I HAD to erase and install when conflicts with Leopard literally ate my applications and information (I watched them disappear from the screen before my eyes - poof! gone). I nuked all three RAID drives from Disk Utility (since Leopard would not cooperate with the option that it claimed was there of nuking them for me).
I got a new and different screen several hours ago, after many successful boot-ups with Leopard. That screen showed a question mark in the middle of a box on a gray screen. I decided after the third time of rebooting and getting the same thing, that I would just let it go for a long time and see what happened. It ended up resolving to show Finder in that little box in the center, and then FINALLY, it put the system into true log-in mode (btw, my cursor did not work at all during that time - annoying).
The error I received after that may or may not shed some possible light on what could have happened. I updated Symantec (Norton) Internet Security for Mac right before that (not realizing that most of the features are still unsupported - grrr!). I was able to load the AV portions after doing a manual update, and after the attempts at getting some protection going, I got an error message saying that some of the recent installations may be causing conflicts.
The system has been vastly slower to boot up since installing Leopard, period. But once I tried to update the Norton AV, that was when I got the bizarre start-up screen. It took approximately 20 minutes to run to get past that screen then. Since that though, I've logged back in successfully. On another interesting note, it seems that the HD I end up logging into isn't always the one I've clicked on the Start-up Disk. Hey, it's an adventure at this point LOL.