SMC Reset worked for me... double login
MacBook 10.5.1 installed last weekend >> 2 days later noted MB was hotter than hell in my bag. Pulled it out and battery nearly drained (closed it with a full charge and Airport on). Then noted that when closed, it wouldn't sleep but light stays solid and I can hear the HD occasionally kicking on (the same sound I usually hear when I open up the MB from sleep by lifting the lid). It seemed to be stuck in a loop.
After reading this and other forums with same issues, tried SMC reboot (take out battery, hold power button 5 seconds, replace and reboot). All is as it should be now, with minimal delay on awakening and prompt (5-8 sec) sleeping on closing. Hopefully the problem won't come back in a few days.
FYI, I was also getting an odd problem where, on awakening and getting the password screen (I have it set to enter password on awakening), I would enter password, then my desktop would show for a split second, then I would get password screen again and have to re-enter before I got to my desktop for good. This has gone away now, but I mention it because I haven't seen anyone else describe this behavior.