Crack it - if you can either drill or saw through the post where the seat clamp is, you could do a number of things. Either the post will collapse at that point and you can get it out, you might be able to pry it off with a screwdriver, or you've at least removed some weight.

Also, start pouring chemicals on that sucker. super-lightweight oil/lube, penetrating oil, acetone, rubbing alcohol, all these things could contribute to solving your problem. Just, uh, make sure it's well-ventilated.

Careful work with a propane torch has worked in the past for me, but it made me so nervous I'm never doing it again.