I bought a vintage Trek frame a few weeks ago only to find the seatpost severely stuck in the frame. The guys at the LBS put it in the table vise but it wouldn't budge. I finally got it out by soaking it in ammonia and then using a pipe wrench. Here's what I learned here and elsewhere about stuck seat posts:
* Pay attention to what metals you are working with. I figured my post was aluminum because a magnet did not stick (though apparently this is not a foolproof test). The LBS guys heated up my post which would actually cause it to expand (in a way this could still be good after the post cooled down, maybe?). Apparently with steel frame and aluminum post you want to cool everything and then heat the frame because the steel will expand faster than aluminum (saw that on a post here I believe). I eventually removed the seat post without using any heating or cooling.
* Sheldon is right that ammonia eats at the corrosion that forms between steel and aluminum. I turned the bike over and poured the ammonia (bought it at Kroger grocery for $1.99) into the seat tube from the bottom bracket area. I was able to fill up the whole seat tube until I could see from the BB hole that it was full (it leaked out very slowly if at all -- luckily my seat post was impermeable and I did not have to use any tape). I let it sit for about 48 hours, occasionally banging a bit with wood on the seat post and seat tube hoping to move things around a bit.
* Sheldon might be wrong when he says not to use a pipe wrench. A pipe wrench, I found out, is designed to tighten onto a round pipe as you pull. I was finally able to free my post by putting the pipe wrench on there, laying the bike over, and, using the ground as leverage against the end of the pipe wrench, standing on the whole thing so that the force of my weight downward turned the wrench.
* There's this place called Harbor Freight that has ridiculously cheap tools and other hardware store items (thanks to the LBS for this tip!). If there is one near you I would definitely check it out:
http://www.harborfreight.com/cpi/ctaf/retail_stores.taf
* Word, thanks to everyone else who posted on stuck seat posts and helped me free mine!