For those who purchased the Epic Edition of Bulletstorm, the Gears of War 3 beta launches today. However, for people who still want to get in on the action, but didn't purchase Bulletstorm, there are still ways in order to gain access to the public beta next week.
One way is to pre-order the Standard, Limited or Epic edition of Gears 3 from certain retailers depending on you region. The selected retailers are GAME and Gamestation for the UK, Gamestop in the US and Canada, and EB Games in Australia and New Zealand.
The beta will eventually include 4 maps (Thrashball, Checkout, Trenches and Old Town) and 3 game modes (Team Deathmatch, Capture the Leader and King of the Hill). At time of writing, only Team Deathmatch and the maps Thrashball and Checkout have been released. The beta also includes plenty of unlockable items, such as varying skins for weapons and different characters to play as.
If you aren't in already, the public version of the beta (for pre-ordering) begins on the 25th of April and concludes on the 15th of May.
Source: Gears of War Official Site