As Phoenix is as yet not officially announced, no release date has been set.
Rumor has it Phoenix will be released before Halo 2.
UPDATE: Since Phoenix has been cancelled, obviously it won't ever be released, and no release date was ever officially announced or even hinted at. The same is true of the as-yet unnamed replacement project that the Phoenix team is working on as of this writing.
These developments also discredit the source of the rumors that indicated Phoenix (or, indeed, any project from that team) would be released before Halo 2.