While on the surface, I don't like the idea of fragmenting OS X into three versions, they could end up very close under the hood (Snow Leopard-based) as well as in the development arena (Xcode with mostly common APIs). And if a tablet ever comes, there really IS a need for something in between Mac OS X and iPhone OS X as we now know them.
The iPhone works so well because it doesn't try to shoehorn a desktop OS and UI into a 3" screen. Apps are MEANT to run at that size, and interaction is of a new touch-centric kind that works great. Meanwhile, current tablets work so badly because they do try to shoehorn a mouse-and-keyboard OS and UI into a stylus interface.
Trying to make the iPhone OS work "as is" for a bigger tablet could be a mistake: the apps SHOULD be different for a different screen size, and the productivity/power could be closer to a desktop Mac, so why waste that? Only you want real touch-interaction that's designed for touch alone (which Mac OS X is not).
So.... a third branch sounds plausible to me. A new OS branch and a new device family (no stylus required) that makes tablets actually useful to the general public--finally.
I'd hope the tablet offered a full OS X mode for running desktop apps in a pinch... but it might be a mistake to release that mode too soon: it would give developers less reason to make REAL Apple tablet apps. And we've seen what great things devs can do with a new platform when they really target that platform's strengths.