In an interview with IGN,
Crackdown 2 producer James Cope has confirmed:

You can definitely expect a demo to hit Live before the game is out but I can’t say when that will be… yet. The Crackdown demo was fantastic and we want to carry on in that form.

As someone who pre-ordered the original game simply for the Halo 3 beta but then was blown away by both the demo and the game itself I am very much looking forward to this.