Well Gametrailers just released a new trailer for Batman Arkham Asylum with the announcement of a demo!
This Friday, you'll be able to download the campaign demo! The demo starts you right at the beginning of the full game taking The Joker to Arkham Asylum and giving you a glimpse of things to come. Try out free-flow combat and stealth, plus unlock some character bios to see a bit of how the game works.
If you can't wait, then here's a video of the first 10 minutes of the game: SPOILERS! Don't watch if you don't want anything spoiled!