I had a similar problem with my car a while back. The trick I used was as follows. Loosen the nuts right off, but don't remove them (leaving three or four threads on), so they restrain the wheel when it comes free. Jack the car up and support it solidly on stands - I also put the spare wheel under the sill on the side I am working on.
Take a 2-3lb club hammer, and gently tap around the rim, working in a semi-random criss-cross fashion, e.g 12 o'clock, 7, 3, 10, 5, etc. If you have enough room, do so from the back as well. This should eventually break the corrosion seal that is holding the wheel on - be careful as it might shift suddenly (hence leaving the nuts on).
Once off, you should be able to see signs of where it was sticking, such as fresh orangey rust on the hub. Clean these areas up with a bit of sandpaper so the wheel is easier to refit. Before refitting the wheel use a bit of copper grease on the mating surfaces to prevent the problem from re-occuring, but do not put any on the studs/nuts or anywhere near the brakes!