According to the official Halo website , the new Halo: Reach Beta which will be available to all those who bough Halo 3: ODST, will start on May 3, and will end on May 19.
Bungie will be turning on different game modes throughout different dates:
May 3 – “Arena mode”, “Free-for-All” and “Grab Bag” playlists, “which include a mix of classic and brand new game types.”
May 7 – Spartan vs. Elite “Invasion” playlist.
May 14 – “Generator Defense” game type.
And one last surprise:
“Everyone who logs in and plays at least one full game of any type in the “Halo: Reach” beta will receive a free “Halo: Reach” Avatar hoodie via the Halo Waypoint portal on Xbox LIVE. To redeem the hoodie, players must access Halo Waypoint and browse their “Halo” Career statistics after completing a match. Additionally, as a way to give even more “Halo” fans a chance to join the fun, online multiplayer for the “Halo: Reach” beta, “Halo 3: ODST” and “Halo 3” will be free for all Xbox LIVE members during the “Halo Multiplayer Unlock Weekend,” which runs from May 14 at 9 a.m. PDT through May 17 at 9 a.m. PDT.”