I'm sure it will be a pack-in with a game, a-la the Wii Remote/Wii Play bundle, or the Zapper/Link's Crossbow bundle. Also, so long as it's cheap and built-into new versions of the controller, I'm sure it will be supported. Three years from now the vast majority of controllers out there would be MotionPlus enabled.
Also, I think that the old logic of accessories being doomed to failure has kind of disappeared, probably starting with the RAM pack on the N64. The market has just expanded so much that these things are becoming more and more viable. Look at Rock Band, Guitar Hero, and Dance Dance Revolution, for example. These (especially the first two) represent some of the biggest success stories in console gaming right now and they all require special peripherals to play. If Nintendo bundles this with something good (Wii Sports II, perhaps?) for a nominal price, it's sure to take off. Hopefully it will also help with some older games, like boxing and putting in Wii Sports.
Can't tell for sure from the pic... is there a passthru port on that for the nunchuk?
It looks like there is a passthru with a door on it, but even if there isn't, the answer to your question is still yes. Maybe we're looking at a prototype that doesn't have the passthru implemented, but, yeah . . . it will have a passthru. How could it not? It wouldn't even begin to make sense to make something like this but leave off the passthru. That makes me think of something kind of interesting, though. When you have this, the motion sensing on the remote will (supposedly) be perfect, but the nunchuck will remain lame and frustrating. Probably not a huge issue. Developers will use the remote for things that need precision and reserve the nunchuck for rudimentary stuff like lobbing grenades. Still, it will be kind of funny to have half the controller finally work while the other half remains broken.