Check out the CZ Kadet .22lr pistol, or buy a CZ 75/85 or an SP01 model, and then buy the Kadet .22lr conversion slide for it. I own a CZ with the conversion slide, and it is reliable and accurate, not to mention that it feels realistic to shoot when mounted on your CZ 9mm or .40 pistol frame.
The new issue of Shooting Illustrated magazine (I read it from a rack at a local store, don't have it here with me) has a review of leading .22 conversion slides from companies including Advantage Arms, Kimber, Glock, and the CZ Kadet. The Kadet came out with the best accuracy based on an average of multiple 5-shot groups at 25 yards, with several leading ammo types, although one of the Advantage units was a very close 2nd. That doesn't surprise me, as I've found the Kadet to be the most accurate .22lr pistol I've owned.
I have to say, that if a .22 conversion unit for the M&P existed, I'd buy and use it in a heartbeat. But until that's available, or as a very nice-shooting alternative, you may want to at least consider the CZ Kadet pistol or the conversion unit.
Here's a picture of the full Kadet pistol. The conversion unit, mounted on a CZ 75/85 or SP01 frame, looks similar.