Just in case nobody has mentioned, the SiB can play video in fullscreen by pressing f, as it's actually a frontend of mplayer. Therefore, you must select mplayer as default player in SiB to use the hotkeys in mplayer.

Also, in case you don't know, you can even do something else with your N900 while it's playing video fullscreen on TV. That's tip #44 of the N900: Useful Tips (a summary).