I am running out of ideas but since you are logged out when the computer wakes up that would seem to indicate a high probability of some external influence triggering the wakeup. This will take some time but you might try disconnecting external connections one at a time and see if and when the wakeups stop. I would start with being sure Airport and Bluetooth are both turned off in System Preferences. From there I would progress to disconnecting the ethernet cable, then anything on the USB port(s), then the firewire port(s). If your computer still wakes with all the external devices disconnected and/or turned off then it has to be something internal and it will probably be a real pain to isolate.
