I have Mac mini with the latest Leopard + all patches. I've set it up for the monitor to go to sleep after 5 minutes of inactivity, but it just won't turn off.
The trouble started last week after I upgraded the RAM from 1gb to 2gb. Software-wise nothing changed.
Oddly enough, I have the same exact problem on my pc laptop. I suspected that the cat would just jump on the keyboard in the middle of the night and the monitor would wake up. But I locked the cat out of the computer room for the night and the monitor was still on in the morning. The cat denies all knowledge.