I had a problem with hibernating a Windows XP system containing an on-board Intel(R) 82566DC Gigabit NIC. The hibernation was ok, but it would wake up in a couple of seconds after shutting down. Today I looked around a little more and found the following setting: "Wake On Directed Packet". Unchecking it solved the problem.
To do this, go to your network connections folder, right click on your connection and select properties. Press the "Configure" button. Go to the power management tab and uncheck the box. Warning! Changing this setting will temporarily interrupt your network connection just like when you disable TCP checksum offloading. Also, probably this option is available on other network cards too, not just Intel ones.