http://blog.nella.org/archives/2006/01/google_video_ha.html
quote:
When you click to see a movie in the Google Video Player, you download a tiny text file (with file extension GVP). That causes the Google Video Player to launch. It reads the file and starts to download the video via HTTP, just like the Flash player is doing in the web browser. The GVP file is just a text file, so you can open it and find the URL. If you fetch the URL yourself with wget (i.e. no cookies, no javascript, etc) you get a big binary stream. The content-type claims that it is "video/x-msvideo", but it does not play in Windows Media Player.
Site also contains a perl script to turn the URL in the file into a playable file - but apart from that, download the AVI and play it in VLC.