[houltmac]

Since they are both desktops... could it be your modem/router? If both machines are attached via ethernet then it could be the Wake for Ethernet network administrator access option in System Preferences>Energy Saver>Options.

Just another quick and simple one. Other than that, could you list the third party (not Apple or system stuff) apps that are installed on both machines, including preference panes and haxies? Sounds like it would be something they have in common most likely.