nope. p226 9mm mags fit in the 9c that i have, never had the chance to try it out in the fullsize. p226 40 mags may or may not fit. functionality is a different story.
I bought a 226 magazine up at sig last week. It fits and feeds "most" of the time. Surprisingly the 9mm magazine was feeding better with .40 ammo than the .40 magaine was. This will be added to the pile of M&P projects that I don't really have time for right now. Going to see if I can make a new mag release that can hold the magazine up higher in the magwell without having to alter the frame of the m&p.