I'm thinking 10.5.7 is one of those OS X point updates from hell. It smells like a bridge to 10.6, meaning it breaks quite a few things but will be supported until 10.5 expires.
I think it's broken the Airport Extreme base station to Airport Express peripheral network bridging as well as some printing and faxing peripherals.
The latest update bug I've found is related to screen locking that occurs during the update process from 10.5.6 on a PPC iMac. If a screen saver locks during the update, you may get a view of the screen cursor but you don't get an unlock dialog.
The iMac appears to be unable to awake from a sleep state.
If this happens to you I recommend letting the machine sit for at least 30 minutes. If you don't hear any disk activity then power cycle it. I did that and the update appeared to complete normally.
I also recommend all peripheral drives be detached during the 10.5.7 update. That's always a good recommendation, but it's easy to forget.
Apple's software quality has passed through the toilet stage and is reaching for the sewer stage!
See also: Requiem for a gerserker (geek berserker)