Well this is not official, but in general firearms lifetime warranties are to the original purchaser, but the company will perform any reasonable service for the life of the firearm because it's an inherently dangerous item and they do not want to be held liable for returning to you something that is known to be defective in design or manufacture.
Unlike the firvilous lawsuits brought against firearms manufacturers, that would be flirting with a legitimate product liability suit.
How far a manufacturer will go varies. BAsically all with lifetime warranties will fix something minor if parts are still in production (i.e. broken extractor, etc). IF it is major, some may offer you a heavily discounted refurbish service rather than fix it straight out (i.e. you shot it enough to wear out the barrel). If the parts or gun are no longer being manufactured, and the damage is from normal use, they will often offer to replace it with something newer if you don't want the broken gun as an heirloom (once again possibly charging you). If you keep it they will usually issue you a letter stating it is broken and not to be used.
For something as new as the M&P, I doubt there would be any issues. THe only possibility would be a major lemon that had been back and forth between S&W and a PITA customer with a bad attitude.