At some point today, a Tribes:Vengeance demo will be released, it apparantly weighs in at a not so tiny 374mb, and includes a solitary singleplayer mission.

I still can't get used to a Tribes game having single player missions, not sure whether to give it a try or not.

**Update**
As expected the demo was released, links below;

http://www.3dgamers.com/news/more/1094835612/
http://www.fileshack.com/file.x?fid=5410