Helio Ocean, Windows Mobile 6 Standard, and UMTS? This here Pantech C810, naturally. Boy Genius Report has pictures of the AT&T-branded device being called "Mustang" and "Duo" internally, though it's not known whether either of those names will make it to production; features are said to include a 3G radio of some sort (either UMTS or UMTS / HSDPA), microSD expansion, that wild dual-slide action with a numeric keypad in one direction and a full QWERTY deal in the other, and a 65,000-color display -- but other than that, we're in the dark. It's rumored to be hitting as soon as next month for $250 on contract, and if this all checks out, we suspect AT&T may have a difficult time keeping 'em stocked.
Update: We've heard from several folks that this device may actually be the C810, which would be more in line with Pantech's naming convention for AT&T devices. We'll keep you updated.