Solved. I discovered a wake timer event in System event log:
The system has resumed from sleep.
Sleep Time: ?2010?-?03?-?16T04:51:52.023330900Z
Wake Time: ?2010?-?03?-?16T07:04:08.353048800Z
Wake Source: Timer - Windows will execute '\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.
This alerted me to the existence of many scheduled tasks--most of them hidden--that I disabled or rescheduled in Windows Task Scheduler to stop the PC from waking up in the middle of the night. You can also control whether events are allowed to wake the computer. Note that, for some reason, disabling wake timers in Windows 7 advanced power management didn't stop these tasks from waking my PC.
Update Check (changed from daily at 2 AM to every 4 weeks at 2 PM)
Customer Experience Improvement Program
Defrag (changed to 7 PM instead of 2:30 AM)
Diagnosis (changed from 1 AM to PM)
WinSAT (changed from 1 AM to 1:30 PM)
mcupdate_scheduled (changed from 7:13 AM to 11:13 AM)
Power Efficiency Diagnostics
AnalyzeSystem (changed from 6 AM to 6 PM)
RegIdleBackup (changed from 12 AM to 12 PM)
SvcRestartTask (changed from 12 AM to 12:15 PM)
SR (changed from 12 AM to 10 PM)
SynchronizeTime (changed from 1 AM to 1 PM)