Sili-kroil. Best stuff on the planet. No, it ain't cheap. But it's the secret to working with bolts that have been rusted in place for 50 years. Heat up the oven, spray with the Kroil, and wait for things to cool enough to handle. It'll come off like a charm.

Here is a link that might be useful: Sili-kroil

Holly's idea of heating up the oven is a good idea. Might expand the metal enough.

I got my screws out. They were in there pretty tight but they came out fine.

Another idea: use a socket wrench with a philips screwdriver head on it (make sure it is a snug fit in the screw head). Apply heavy pressure on the wrench head, making sure you don't slip and strip the head, and slowly turn the wrench. I bet it will come right out.

-Stooxie

I like stooxie's idea. In addition, there are products to help prevent the screwdriver from slipping and stripping the head. Basically it is just abrasive particles (SiC, diamond, alumina?) suspended in an oil. The particles bite into both the screwhead and the screwdriver. Mine is called "Screw Doctor."