Wizards Of The Coast have released a Test Drive pack for download from their website.
It contains a simplified rules set, free scenario and a limited version of the character generation tool (1st to 3rd levels).
If you ever listened to the D&D Podcasts with the guys from Penny Arcade, PvP and Wil Wheaton then it's the scenario used for those first podcast recordings.
http://www.wizards.com/default.asp?x=dnd/4news/20090428 has all the information.
If you fancy playing the game then come along to the next D&D Day and check out the other thread.
Cheers,
Dave