Looks like the Wii MotionPlus add-on will indeed be shipping this June - June 8th to be exact - as Amazon is currently taking pre-orders. The device will cost $20, which is about $10 too expensive, considering it's a feature that should have been implemented into Wii remotes from the get-go.
But you know what they say: “People buy a lot of accessories for the Wii and often those accessories tend to be a bit overpriced, but people still buy them anyway.” I think that's how the saying goes.
If you're planning on buying Wii Sports Resort at $50, it'll include a MotionPlus add-on. However, it looks like EA Sports' Tiger Woods 10 and Grand Slam Tennis will NOT include the MotionPlus sensor as both items indicate that it's sold separately. Scratch that. Bundles will be available. You can get Tiger Woods 10 with the MotionPlus add-on at Newegg for $56.99 - apparently Grand Slam Tennis is still a standalone title ($46.99), though.
I'm a big, fat sucker who's a glutton for punishment guy who'll get an okay deal after all, so I'll eventually be laying out a grand total of $70 $57 for Tiger and the MotionPlus doodad.