After install: stuck at Apple logo and spinning progress indicator...

The first time around, the errors reported seemed to be rather severe, the kind DiskUtility didn't want to attempt to repair (forgot the precise message). DiskWarrior fixed these issues successfully. Also, prior to the install, the disk checked out fine. So whatever issues arose were due to the install, and/or reboot attempts.
The second time, there wasn't anything to repair (DiskWarrior did the job), I just wanted to make sure DiskUtility agreed with that assessment). I even repaired permissions before the second install attempt, and there two minor permissions problems showed up (postfix related, which is essentially unused on a OS X client machine anyway)
Now, of course, I have to force a shut-down each time I try to regain control over the machine, so I run DiskUtility before I attempt any other operation to recover from that (even though the journal should prevent disasters) and the file system checks out fine each time (as it should).

The drive is a 500GB drive, with somewhere between 40-50GB free, which should be adequate (the 10-15% thing is a bit a thing of the past when drives were smaller, having 5-10x the free space requirements of the install and 40x the amount of what is likely the biggest single file on the drive, should be ample)

So thanks for the concern, but the sort of obvious things like these I think I got covered, after all, this is about the 10th major revision of NeXTSTEP nee OS X I'm installing on various computers.

I remember with Leopard there somewhat similar issues related to an incompatible version of APE/Haxies, but a) not even SafeMode boot works (and APE/Haxies aren't loaded in SafeMode), and b) Apple supposedly moves dubious software out of the way, and they are certainly aware of APE/Haxies given how widely these are used.

Also, it seems from skimming the forums here, that I'm not the only person with the same or similar issues.

One issue that comes to mind: does 10.6 (unlike 10.5) do anything relevant with the EFI partition? If so, maybe that EFI partition has become corrupted, but since DiskUtility so nicely hides the existence of this partition from users, it might be somewhat difficult to fix such a problem, if that truly were the cause. Plus, of course, one would hope, that the installer would nuke/recreate the EFI partition as needed, curing such potential issues.

Verbose mode boot (v key) doesn't seem to work, either, so there are no meaningful diagnostics to be gathered. So maybe setting the boot-args value in pram may be worth a shot...

Well, I went ahead and used
nvram boot-args="-v -x"
to force verbose, safe-mode boot.

The boot gets stuck right after the message

DSMOS has arrived.

DSMOS of course stands for "Don't Steal Mac OS". That means that
a) either Apple's anti-piracy provisions don't realize that this was an upgrade on an original MacBook Pro from an original Apple SN Retail DVD of an Leopard install that was made from an original Apple install DVD, OR
b) whatever comes directly after the copy protection stuff gets loaded hangs the system before it outputs any message.

While that narrows down the issue for an Apple engineer familiar with the boot process, it of course doesn't help me much, lest I were to try installing some Hackintosh kext, but I really don't want to have to hack an original Mac....

Here som additional data points. I proceeded to wipe the disk clear and install SnowKitty from scratch. That went without a hickup. Then I used Migration Assistant to transfer my programs and settings and data onto the new install from a TimeMachine backup.

Result:
a) MigrationAssistant got stuck transfering NetworkSettings or something like that for well over 7h while showing 7 minutes left on the transfer for the entire time. Eventually I had to quit MA.

b) upon rebooting: same issue as before.

Result: something in my programs and settings is making 10.6 choke, but it's not clear what.
Worse, even if I were to bite the bullet and started installing everything from scratch, I would have to do a full reboot after each third party software I install, to see if that prevents the system from booting, then noting that software as off-limits, erase the disk, and start again from scratch, without that software, and continue until I'm done with each potential offender. This could take days, or weeks to complete. Obviously, that's not a viable option, at least not as long as I'm not getting paid to do debugging for Apple...

So at this point, I'm in the process of giving it one more try, installing from scratch, transferring only data and programs, but not the network settings, from the old backup, just in case some settings are what's causing the problems; the settings are easy enough to recreate. But if that also fails, SL is a non-starter, because there's currently no viable way to figure out what is the problem, since neither an upgrade nor an install plus MigrationAssistant lead to a working setup.

I may be getting a little bit closer to what's the deal here, but not close enough to have a solution:

Since I previously first installed from scratch, and then manually called up Migration Assistant, I now tried to migrate data and settings directly during the install.
This time I was lucky to be at the computer at the right time: I was down to 4 minutes remaining, when time zone, then computer and finally network settings were about to be transfered.
As it got stuck with network settings, the time slowly went up from 4 minutes to 7, and there it stuck.

So it's the same 7 minutes indicated before.

Needless to say, when I force rebooted the machine after several hours of "7 minutes waiting", it exhibited the same problems.

I will now try a last time, just transferring data and user settings, but not transferring computer settings, just to see if that helps.

In any case, to some nay-sayers who replied to some comments of mine in other threads: yes, this is Apple's problem, because at that stage during the boot no third party thing should be loaded that should not be defeatable with safe-boot, and safe-boot won't help.
IMO, it's Apple's responsibility to make an OS that as long as the executables that Apple supplies are present and not corrupted through hardware issues, the system will boot into safe-mode no matter what. If settings are missing or settings files are corrupt, they should during a safe-boot either be recreated or replaced temporarily with hard-coded defaults that allow the system to come up to the point where the user can take control.
It's not acceptable that some sort of incorrectly upgraded or transferred or otherwise corrupted settings or preference file prevents the system from going on-line.

Worse, I have a sneaking suspicion that if the user manually sets the password for the System.keychain, the migration software has trouble, but that's just that, a sneaking suspicion, because that's just about the only "non-standard" thing in my setup, and that's actually something any decent sysadmin would do; after all, why would you trust a system that holds key data (pun intended) guarded by a key that you don't know, can't retrieve and might possibly be accessible by some sort of undocumented back-door.

Anyway, drum-roll, we'll see how this renewed attempt works out in 5h from now or so.
If things work then, I'll have to redo all my settings, which is annoying but doable, if not, it's back to 10.5.8 for the indefinite future.

This is likely not the problem. On my machine EVERYTHING is current, I even have the 9L31a not just the 9L30 build of 10.5.8 on the system before the upgrade.

Likely it's something in the Network or Machine settings that the migration scripts can't handle, e.g. stale information, special characters, too many locations, etc.

As far as getting the disk out: force power down the system, then boot with the mouse or trackpad button held down (and I mean press it and keep it pressed while turning the machine on and until the disk is ejected)