There are several reasons I can think of for why we haven't seen this released in retail form yet.
Real-d, which Cameron is on the board of directors of, was going to partner with samsung on a tv that would've given us the best of both worlds: active polarization full-resolution 3d panels that use passive glasses. The deal fell through late last year and there's been no word since. Panasonic's active sets were probably considered "good enough" by Cameron to get the exclusive, but I'm sure he's waiting for 3dtv tech to catch up before he releases Avatar into the wild.
This recent announcement that the bd3d of Titanic will be spread across two discs has me wondering if the same thing will be done for Avatar. The theatrical cut, at 162 minutes, fit on a single disc and got rave reviews. The longest version is 178 minutes, not as long as Titanic's 191, but still pretty long. Considering AVC encoding tends to be more bit-intensive, they might not be able to fit it on a single disc. Add to that the fact that many people already have this movie on standard 2d blu-ray (some of them twice) and you can see why Fox might want to wait a while. Titanic has the advantage of not having been on blu-ray at all until the same day as the bd3d release. Avatar doesn't have that advantage.
As for Panasonic's exclusivity deal having expired, well, Fox was never going to make an announcement the very day after, so no surprises there. I am kinda shocked that we're getting the official announcement of the post-converted Titanic before the natively-shot Avatar, though.