I think it'll be a system wide API (similar to pressing the Home button) to switch the main display to the tablet. Of course, then the problem becomes, if you do that, what happens to what you were already displaying there? For the Zelda demo, they put an overlay on the action. I don't think that's a good solution.
I imagine most devs will use it like a DS, or rather 3DS which even more strictly uses the bottom screen for secondary functions.
I'm also concerned at just what kind of weirdness local multiplayer games will introduce. How will they get across to consumers that Game A needs one tablet and four Wiimotes, and Game B needs two tablets and two Wiimotes, etc.? It's a slippery slope having a controller that's limited to one or, at best, two players, and then relying on the previous console's controllers to supplement the rest. (On the other hand, Find Mii et al were to my knowledge, the first local multiplayer console 5-player games.) I'm looking forward to having many of these questions answered at E3.
