My favorite shop here in Phoenix (Slippery Pig Bike Shop, you can find them in the LBS thread) regularly fixes flats on Magnas and Pacifics and other Wal-Mart bikes, always with the same respect and friendliness they show to the carbon fiber crowd (and the 10-inch travel FS crowd for that matter). They're also very fixed and SS friendly, unlike almost every other shop.
Oh yeah, this is my first post. Please go easy on me, even if I do like pie.