The beta can currently be found on the Market by searching for Angry Birds. It's quite a fun and addictive game. The objective is to launch birds from a slingshot and kill the green pigs on each level.
The green pigs are hidden in a structure, so launch the birds to knock the structure down, and try and kill the pigs by launching a bird directly in to it, or knocking the structure down on top of it.
The game plays perfectly fine on my Desire, and have heard it plays fine on other phones, but the Evo 4g apparently has problems with it and the app apparently keeps crashing on it.
Definitely a game worth trying.