The Windows Ultimate extra, DreamScene has been pre-released and the performance on it seems to be spot on. On my laptop, which is admittedly a pretty beefy machine, I don't see my CPU spike at all when I have dreamscene enabled. Here is short description of the Ultimate Extra from the Windows Ultimate site:
Your desktop background comes to life with Windows DreamScene™. When you download this Ultimate Extra, you can select a video for your desktop background the same way you select a stationary picture. You can choose one of the high-quality looping videos that we’ve included, or use a video from a company that produces content for DreamScene. You can even use your own video as a background.
If you’re wondering what effect a video background might have on your battery life, don’t worry. Windows DreamScene respects your power settings and pauses the video to conserve battery power.
Also, you should check out the WMV HD Content Showcase videos as a few good ones to start with. Lots of good 720p and 1080p videos there for you all out there with nice screens.