I just had to deal with the same issue over the weekend. Make sure you take the caliper off first.
Spray some PB Blaster (or equivalent) into the threaded holes and around the studs. Thread bolts into the two holes, tighten them up as much as you think you can get away with, then hit the "hat" part of the rotor with a 3 lb short sledge hammer while rotating the rotor. You want to set up vibrations in the rotor that will loosen-up the corrosion. Keep tightening up the bolts.
When that doesn't work, start whacking the back of the rotor with the sledge through the gap in the backing plate where the caliper used to be. Again, keep turning the rotor while hitting the back and attempting to tighten the bolts.
You have to be careful about hitting the rotor too hard. You can damage the wheel bearings.
A fall-back option is to use a sawz-all with a metal cutting blade to make some cuts in the rotor, then split the rotor using a cold chisel! I once had to do this to the rotors on my old DSM.
When you get the f****er finally off, brush the corrosion products off the hub mounting face and apply some anti-seize before putting the new rotor on.