Here are some of the posts I promised about stuck seatposts - there are many more! Also my own method is described below - it hasn't failed me yet, but you never know!
http://www.bikeforums.net/showthread...ck#post6696663
http://www.bikeforums.net/showthread...st#post5733521
http://www.bikeforums.net/showthread...st#post2673479
http://www.bikeforums.net/showthread...ck#post7218699
http://www.bikeforums.net/showthread...ck#post1765339
http://www.bikeforums.net/showthread...stuck+seatpost
Spray WD40 around the top of the frame where the seat pin goes in. Remove the seat binder bolt completely and if there's a slot in the back of the top of the frame's seat tube spray WD over that as well (It's a good idea to cover the tyres and rims to protect them with so much WD40 flying around). If you can, remove the bottom bracket, turn the bike over and spray WD40 up (down) the seat tube from inside, Leave it 20 minutes or so to soak in then spray again. Get a long pole (I use a metal pole or a broom handle, but the stronger and longer the better) to use as a lever and shove it under the saddle from behind so when you push sideways it presses against the side rails of the saddle.
Now get a big, heavy and strong assistant - (and if you ask the wife or girlfriend {not both at the same time!} change that to 'beautiful and intelligent' assistant when you ask her, for safety reasons!) Also get a large hammer (don't panic!) and find your can of WD40 again.
Get your assistant to stand next to the bike facing across the frame and to push down hard on the top tube with both hands while you push and pull sideways on your long lever, backwards and forwards with both hands. Make sure your assistant is ready and don't try to do too much at once. You're both going to need to rest from time to time so agree to say when either of you needs a break, so nobody suddenly lets go without warning.
If the saddle moves from side to side even a little, great - keep pushing and pulling and start to put upwards pressure on your long lever as the arc of movement increases. The arc of movement will be tiny at least to start with. If it doesn't move at all or the arc doesn't increase, use your big hammer to tap the far end of your lever sharply forwards then backwards until it does; then proceed as previously described. Don't hit the saddle or frame with your hammer, only the far end of your lever, keeping the lever under tension as you do this. Spray more WD40 from time to time as the seat post very very very gradually comes out.
This took me about an hour and most of a large can of WD40 that I had to wipe up afterwards, but saved an expensive frame, the seatpost and the saddle. If you can fit an old saddle so much the better, just for this job. As the post comes out it gets easier - it's really hard at first and you need to put all your weight into it. Eventually you can dispense with the lever and wriggle it out by hand holding the saddle. When it's out, clean up the seat post and the inside of the frame and grease the seat post with a suitable anti-seize or grease before replacing it.
Good luck! If this doesn't work - and it always has for me, try some of the more drastic and destructive methods described elsewhere.