I'm running Vista x64 SP2 and Windows Media Player 11. I have an NVIDIA GeForce 8600 graphics card. I also make sure that the "Allow screen saver during playback" is unchecked in WMP -> Tools -> Options. After 20 minutes my screen saver kicks in.
Anyhow I've solved the problem with the screen saver. I like to keep my system as updated as possible, so I install NVIDIA's drivers directly from their website. Yesterday I removed NVIDIA's drivers, and I let Windows install it's own drivers for the graphics card. Of course Microsoft is just packaging a form of NVIDIA's drivers, but now my screen saver stays disabled when I play a movie.