AMD likes to be very hush-hush until the product actually releases.
The code names for the 6000 series GPUs were revealed late august of 2010, launched late October. The 5000 series code names were revealed in July of 2009... released late September. Both launches occurred two months after their codenames were discovered.
The 7000 series codenames were revealed through Catalyst early July.
If it were to follow in the footsteps of the last two launches, it would be a September launch.
Pictures of the 6870 surfaced only two weeks prior to its launch... there's not much notice given before the products hit the shelves. When things start moving, you'll know about it.
Really, there's no way to know for sure. A September launch would tie in nicely with the rumored launch of Bulldozer.