Apparently the demo is being released on different dates, depending on your Xbox Live membership type.
Gold Xbox Live: January 29
Silver Xbox Live: February 6
Note that for those that pre-order the game, they should have received a demo disc, as well as a mystery "premium" item (see Dinosaurs Invade Early With Turok Demo Disc).
The demo has roughly 30 minutes of gameplay.
We're planning on running through it today, and should have our impressions up first thing in the morning for everyone.