In the most recent IGN podcast, Matt stated that Nintendo is having a little trouble with Motion Plus development and will not be out until at least June. Check out the snip-it from the podcast below.

“When I got wind that the DSi announcement which turned out to be pretty accurate…At the same time I heard that Nintendo was internally struggling with Wii MotionPlus and trying to figure out a few hiccups…And that they were going to delay the release of it all the way past June. And also Wii Sports Resort which would be like an eight game title, it would have eight games in it, not ten, and that would also be delayed a little bit…The only thing I got was vague and was like, ‘Oh, they’re having some hiccups in development.’” – Matt Casamassina, Nintendo Voice Chat podcast 39

If you remember when Motion Plus was first announced, it was supposed to come out sometime the next few months in the Spring.