This is in no way a VNC problem, but is relevant to Windows machines in setups which typically use VNC.
I wanted a machine used as a peer-to-peer file server and also used to run processes remotely via UltraVNC to wake whenever invoked from another machine, then go to sleep after a period of inactivity, looking "always on" to other machines on the network while actually saving power. I was originally using an Intel NIC with "wake on directed packet" (essentially wake whenever invoked) enabled. This was never possible due to random wakes, and I disabled "directed packet" wake. "Wake on magic packet" worked fine, I had to wake the machine (by clicking on an icon which called an appropriate batch file) whenever I need to have access to files on it or make a VNC connection, either locally or over the Internet. I then upgraded the motherboard to one with a Realtek NIC, which slept and woke in the same way without trouble.
Recently (June 2012) the machine started waking unexpectedly again. I found that the Realtek NIC driver in an update had a new "wake on pattern match" setting, similar to "wake on directed packet", which was enabled; disabling it while leaving magic packet wake enabled solved the problem.
A question: Does anyone know if the seamless operation I describe can be made to work in any way without frequent unwanted waking? There seems to be too much invisible network activity causing unwanted wakes.