esta en ingles pero miren:
Didn't have the chance to get an early grab at unlocking the Specializations? Good news for you. 343i has announced via their recent Halo Bulletin that the locked Specializations will be available to EVERYONE progressively starting 1/21/12 and spanning for three weeks.
The noted Specializations will be unlocked as followed:
1/21: Pioneer & Pathfinder
1/28: Engineer & Stalker
2/4: Rouge & Tracker