Currently Being ModeratedMay 31, 2011 12:51 PM (in response to Schneb)
Try unchecking "wake for network access" in the Energy Saver prefs pane in System Preferences.
However, that may not do what you want. Since removing any connected device alerts the OS as to a change in config, it will probably wake anyway.
If the USB device is a storage device (hard drive, thumb drive, etc), you should probably dismount it before unplugging it. This necessitates Finder being active, i.e., awake.
Currently Being ModeratedMay 31, 2011 5:48 PM (in response to Don Archibald)
It is strange, though. My iMac at work does not wake when I, for example, unlpug my iPod Touch. I wonder if it is because it is a later model (the aluminum version) than my home iMac (the white plastic version).
I will try the Network Access... it is a simple test.