i have to agree with what was said before about the the aluminum of the stem reacting to seize within the fork. the aluminum oxide that has likely formed can be dealt with by turning the bike upside down, and pouring ammonia into it through the underside of the steerer. let this sit for a good period of time (i.e. more than 48 hrs), and tap occasionally with a hammer to help the ammonia penetrate the interface between the stem and steerer.

after it has sat, pour out the ammonia, and see if you can first twist the stem within the steerer. if it can atleast rotate within the steerer, you're within the clear and it won't be much longer till you can start pulling it out. keep the handlebars attached to the stem to give yourself more leverage.

if you need more leverage still, keep the wheel on the fork. BE CAREFUL WITH THIS however, this is generally NOT recommended unless as a last resort, since using the wheel as leverage can damage both the wheel and the fork through the torquing.

hope this helps, let us know how it goes