You can sign up to the DOTA 2 beta right now via the official web-site.
The beta is a closed beta, so there is every chance that you may not get in, but some people will so be sure to try your luck!
The Closed Beta will begin this Fall 2011! Although no exact date has been revealed yet.
DOTA 2 is the Valve co-developed, Source Engine-powered, IceFrog-created sequel to the internet phenomenon that is Defense of the Ancients. Grown out of a Warcraft 3 Custom Map mod and into a genre all its own, many are extremely pumped about this title. Which keeps most of what you loved about the original game in tact, while making it all prettier and more efficient.
Here is video coverage of the Gamescom 2011 DOTA 2 International tournament!