Mini refuses to sleep

My Core Solo Mini is refusing to sleep. When I put it to sleep, it keeps humming for a couple seconds, and then wakes back up again. I've tried ejecting all of the disks and images (except the internal hard drive) to no avail.

Any ideas?

 

I tried disconnecting all of the peripherals, but it still has the same problem.

Right now, I'm running Firefox and iTunes. The only background programs I know about is Growl.

I disabled wake-on LAN, and I do not have any Bluetooth devices paired with my Mini.

 

Thank you so much! After over a week, I have not had any sleep issues. Much appreciated!