The Halo: Reach multiplayer beta is to begin sometime in Spring of 2010, says Microsoft. The game is due on sale on the autumn.
You will need to own Halo 3: ODST as access to the beta is only available via this.
There's also a new trailer for the game available... at the very bottom of this story actually. It features the opening (not in-game) elements of the game with a look at the Spartan III and Noble Team. Enjoy.