My Mac won't go to sleep

Hello everyone.

I have an older Mac Pro, quad 2.66. Running 10.6.5, Bluetooth keyboard, and magic trackpad. My computer stays on all the time and won't sleep by itself. I have it set to go to sleep after 15 minutes, but it doesn't. I've tried unhooking the printer, external hard drive, etc, but still no dice.

Any ideas?