Some Macs will wake up abruptly, shortly after being put to sleep -- apparently due to a bug in the system that's associated with the AirPort wireless card. Specifically if the computer was put to sleep while connected to a wireless network, it may suffer from this problem.
I wrote a free program called Wireless Sleeper that fixes the issue. It runs quietly in the background and will make sure that the AirPort is turned off before the computer goes to sleep. It will also turn AirPort back on when the computer wakes up.
[robg adds: I've never heard of this issue before, and none of our wireless Macs have had the problem, despite leaving AirPort on and connected all the time. Nonetheless, if you are having the problem, this freebie offers a potential solution.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090113133011479