I got thumbnails working in Rygel. :) Right now rygel doesn't generate the thumbnails on it's own but provide thumbnails if already generated by another application (e.g nautilus) and stored in the freedesktop or maemo (they are almost the same) standard location. It works on both desktop and N900 and now that gupnp stack has been released, a rygel release with this and some other cool features will arrive shortly. :)
Now it's very easy for me to find the videos of my friends.