http://www.macnn.com/articles/09/11/02/switch.from.dst.hurts.cpu.performance/
Time changeover generates Snow Leopard problems
updated 03:30 pm EST, Mon November 2, 2009
Switch from DST hurts CPU performance
The flip from daylight savings to standard time has created unusual problems for Snow Leopard owners, Apple's support forums indicate. Some Mac users note that nearing the point of the official changeover, 2AM on November 1st, their CPU usage peaked as high as 100 percent for no apparent reason. Aside from rendering software unusable, such spikes also run the risk of overheating a computer.
The bug is generally said to have resolved itself after the new time was reached. If the problem has not already ended, it can be fixed by temporarily disabling the Menu Bar clock through System Preferences. It is not known whether or not the issue is being corrected for Mac OS X 10.6.2, which should debut through Software Update later this month.
Fresh-Faced Recruit
Joined: Aug 2001
Aha!
So THAT'S what was going on the night before last. I noticed that one of the cores on my older MBP had maxed out, and the windowserver process was chewing up almost 100% of a CPU for no apparent reason.
I'd assumed it was some odd glitch caused by a program I was running, but it went away before I'd gotten around to restarting.
Didn't even think that it might have something to do with daylight savings time until reading this.