iMac display wakes up even when no one is using it
Try this in your Terminal:

syslog |grep -i "Wake reason"


Then you'll get something like:

Sat Sep 08 09:28:12 MacBookPro kernel[0] : Wake reason = OHC1
Mon Sep 10 21:14:54 MacBookPro kernel[0] : Wake reason = PWRB
...


Depending on the text after the =-sign, you have the reason why your Mac went to sleep. And it can have multiple causes:


OHC: some USB device. If you see OHC1 or OHC2, it's very likely a USB keyboard or mouse has woken up your iMac.
EHC: USB interface too, but if you see this, it will likely be a wireless device.
USB: another USB device (drive, printer,...)
PWRB: If your powerbutton gets pressed (very unlikely)
RTC: a scheduled wake-up (energy saver, backups, iCal events,...)


I hope your item is in this list :)