There's a crazy glitch in the Crackdown 2 demo that will allow you to play as long as you want without hitting that pesky 30 minute time limit. Even better - the glitch will work for a co-op game.
All you need to do is start up a game and right when you go to select your agent, disconnect your console from the internet. After following a few prompts you can start up another game and the clock will be frozen at 30 minutes. You can then invite friends in to join your mayhem.
One caveat though is any rooftop or driving races will cause the game to immediately quit - so you'll want to avoid that. This should do well to hold you over for another 2 weeks until the final game hits. Now get to it agents, there's agility orbs to collect.
Update 6/25/2010: It looks like a patch was pushed out for the demo that fixes this glitch.