CNNSI.com has multiple reports that NAVY will join the Big East for football ONLY. You are correct, in that it hasn’t been formally announced. Army supposedly has decided NOT to join the BigEast, and this is what may be holding up the Navy announcement..
They tried to get Army into a Conference for whatever reason (Money?) a while back and that, in addition to forcing a pro offense on them, destroyed the team. From what I know Army enjoys being Independent at this point.
What I have read is that Navy has contracts with CBS for the Army-Navy Game through 2014, and contracts for specific bowl games (if eligible) through 2013 and contracts to play Notre Dame thru 2026 - some or all of these contracts may keep them from joining the Big East until after the 2013 season.