Did you try heating the bolt up with a torch? Acetylene works best but you might be able to get it with propane. The heat will get the bolt to expand, breaking whatever rusted-on contact it has with the threads. Make sure you get the bolt head cherry red before putting the wrench to it and make sure to wear heavy welding gloves in case you accidentally touch it. And obviously don't burn through any brake lines, bushings, paint, etc.
Good luck, I hate stuck bolts!
One more thing. If some jackass previous owner happened to put the caliper bolts on with loctite or something the torch will melt it away.
