I had the same problem and found a solution that worked for me, but it is just a crude band-aid. I deleted all of the IOFirewire kexts in the System/Library/Extensions folder (backed them up first), rebooted, and everything is now working (except the firewire ports).
Before you try this, please verify that your firewire ports work/do not work, how your LED is flashing/not flashing when you try to sleep, etc, and report back to this thread so we can gain more knowledge on why this problem is occurring. If anyone has rolled back to an earlier version of OSX and still has the problem, that would be good to know as well. I'd love to get my ports working again.
Some background information to help with troubleshooting until it is really fixed:
I have a MacBook Pro 15 v2.2. Installed 10.6, worked fine for a short while, then simultaneously lost the ability to sleep and gained an extra 3 minutes or so of boot time (ridiculously long). If the machine was told to sleep via closing the lid or menu command, it would start to sleep -- light turns white with no pulse -- stuck. It cannot return from this state and I had to do a hard reset. I also could not get the machine to be recognized as a disk when in target disk mode. Looking back on it, this is probably because the firewire kexts were having problems.
I downgraded to 10.4 since that is what came with the machine, and had the same problems, including boot time. Also reset PRAM, did the reset with power off, no battery, holding power button for 5 seconds and all the other Mac rain dances I could think of to no avail.
10.6 includes a new sleep feature, apparently. I don't know the details, but perhaps this has caused the problem. I don't know what the relation to the firewire ports is, however.