Matthew,
Again, this started when I installed the combo 10.7.3 update. Unlike you, my screen does dot go to black-except-cursor, but remains at the desktop (or whatever app I'm in), but simply takes 30 seconds exactly to sleep. I do have a Belkin UPS attached, and I'll try to unplug the USB cable and see what happens. Also remember that I've tested this in a new unsullied user, so it's probably not anything I'm running. Another possibility is that I have 2 RAID arrays connected via eSATA, but I had those before the update and they did not seem to affect sleep then. Could be that Apple changed the way they handle eSATA.
I'm on a 2006 Mac Pro (details below).
Would be nice if we could get some help from an Apple engineer on this issue (hint, hint...)
Model Name: Mac Pro
Model Identifier: MacPro1,1
Processor Name: Dual-Core Intel Xeon
Processor Speed: 2.66 GHz
Number Of Processors: 2
Total Number Of Cores: 4
L2 Cache (per processor): 4 MB
Memory: 6 GB
Bus Speed: 1.33 GHz
Boot ROM Version: MP11.005D.B00
SMC Version (system): 1.7f10