This noise is from under the hood, and I can pretty well localize it to the right side of the car and toward the front. My best guess is something that gets driven by the serpentine belt or the belt itself, but I don't know what.
The noise is a whirring sound of low to moderate pitch, which rises and falls with engine RPM. It is virtually inaudible at idle (600-ish RPM), and is best heard at and above 1300 RPM or so. It is drowned out by road noise at any speed over roughly 40 mph. The A/C being on or off doesn't affect it one way or the other.
A quick punch and release of the throttle seems to cause the headlights to dim noticeably for a second or two as the engine returns to idle. I hooked up a code scanner device to the car the other night, and noted that the voltage it read dipped to about 13.5 V with these quick punch/release cycles on the throttle.
This is about the best I can do at describing it. Anybody have any good guesses?