There is more fallout from the current Leopard update to version 10.5.6. Many users are flocking to the Apple support forums to share their tales of woe about Time Machine failures after the update. (Ironically, Time Machine was on the list of items that received bug fixes).
Upgraders are seeing Time Machine stall, fail to identify the target hard drive, lock up while Time Machine is 'preparing' to backup, and other assorted glitches. There is no guarantee that these issues were caused by the update to 10.5.6, but users are seeing a lot of problems in parallel to the software changes.
MacFixit has quite a bit of guidance on the Time Machine issues. One of the most interesting observations is the long pause while 'preparing' to backup. It seems TM needs to do a deep traversal of the data because it sees a lot of file changes to copy, as would be expected with a major software update. The advice is to wait it out and and eventually TM is likely to get going. I had the same issue and eventually stopped the process, erased my TM drive and started again. That was after more than 2 hours of 'preparing'. It looks like I might have been better off to wait.
I had hoped 10.5.6 would fix the occasional TM error I get telling me that the backup failed, but then when I try again, everything is fine. I spent a good hour on the phone with Apple support on this problem some months ago, but didn't get anywhere. Many others have seen this rather arcane message and reported it. Hopefully it will be fixed in the next OS update.
It's good to keep in mind that most people updated without any dire consequences, but there are enough people grumbling that it merits a closer look by Apple.