I haven't posted on this topic for quite a while now, since the EFI downgrade had fixed my beachball freezes.
However, I've done some further testing now and I think I have some interesting observations to add to the discussion.
I am now 100% sure that the freezes occurring on the mid 2009 MBPs are caused by the EFI 1.7 update.
However, I am also quite convinced by now that the reason is not necessarily the increased SATA speed itself here, since I experienced the beachball lockups even with the stock (Hitachi) drive, which is limited to SATA I speed regardless of the installed EFI version, as the problems still went away after downgrading to EFI 1.6.
Now here's what I recently did:
I updated to 1.7 again, and put a Samsung SSD in my MBP.
While I restored from a Carbon Copy Cloner backup, there was not a single beachball or slowdown (and I copied about 100GB of data).
Once I booted from the SSD, however, BANG, there it was again. Beachballing in Safari, Quicktime and VLC, approximately once every 30 minutes.
Strangely, S.M.A.R.T. CRC error counts did not increase (still zero), as was, not so surprisingly, the case with my stock Hitachi drive.
Downgraded to EFI 1.6 again, and, voilà, no more beachballs.
(Apart from benchmarks, in practical usage I have not seen any slowdowns, BTW. Starting XP inside VMWare does not take one second longer. So, the SATA 1 "bottleneck" is actually not as bad as people think it is.)
While I still do believe you guys when you say the cable (shielding/bends?) is to blame in those cases where the drive is completely unusable, I'm by now convinced that there is more to it than that when it comes to the freezes.
I just wish Apple would admit to the problem, pull the EFI 1.7 from Software Updates, make the downgrade tool publicly available and get to work on a real fix (EFI 1.8). Quite obviously 1.7 was just a dirty and untested hack in order to satisfy the media as quickly as possible. Bad move. And so not what I would have expected from Apple.
I don't want to know how many logic boards have been replaced unnecessarily (and without curing the problem), and how many users are experiencing the freezes without having a clue what's happening to them.
Poor show, indeed, Apple. Shooting themselves in the knee here.
And again, thanks a lot to the guy who posted the downgrade image.