The full on 'Tablet Phone' is the next frontier for mobile devices.
It's only logical. More and more, I see people carry their tablets around with them virtually everywhere. Why not just make phone calls on the same device, just using a small detachable bluetooth module? (And by the way, I mean real cell service, not some net-based substitute.)
And if carriers would wise up, they'd make switching from device to device as easy as flipping a switch, and encourage subscribers to have several mobile devices on the same account, including their Tablet phone. Maybe some days I wouldn't want my tablet phone with me, just a normal smartphone. Why not cater to that?
Whoever does it first will make everybody else slap their foreheads in "Why the hell didn't we think of that?" mode.