Currently there are fleets of AMAZING (in every respect bar sound) copies of SM58's and 57's about, an awful lot of people trying to take them into shops for repairs, often sent away to Shure only to find out they aren't real.
A know of at least a few shops who've replaced one and sent the "faulty" one back to Shure to find out that their sneaky customer has obviously bought a real one from them, a fake one from the net then taken the fake in and had it replaced with their reciept.
SO I'd be real sceptical.