I seem to remember that the first time S&W offered their incentive program involving a free magazine with the purchase of a new M&P pistol, the company ended up back-ordered more than 75,000 magazines. Mec-Gar makes a lot of magazines, granted, but that's a fairly healthy back order to try and fill quickly.
Then the M&P pistol line seemed to really take off when it came to sales, both LE/Gov and commercial.
The last time I called back to order some spare magazine springs for the 40c I was told that Mec-Gar hadn't been able to ship any separate springs for that model magazine, yet, just the complete magazines.
The popularity and increasing sales of the M&P model line is making for some heavy demand on S&W and its vendors.