Seeing as how I work in a performance exhaust shop I might have some insight here.
Ok, for those of you who are going to remove them:
Follow Turbo90awd's advice; let the car get to normal operating temp then soak that bitch in PB Blaster (or similar penetrating oil).
Never, I repeat, NEVER try taking the manifold off when the engine is even warm. Everything should be cool to the touch.
If you have access to an oxy/acetylene torch, then heat up the nut, until its red-hot and then try to take it off. If it starts to move, take it off as fast as you can (don't use an impact).
For those of you who broke a stud:
You can try to drill and easy-out. Make sure you use a center punch to indent the center of the stud, so you get a good hole.
Again penetrating lube, see above.
If you don't feel comfortable drilling into your head, take it to a reputable exhaust shop (not midas or another chain, they are usually sad). There they can actually weld a nut to the stud and extract it that way.
If you’re replacing the studs, use stainless (if you don't want to worry again). Instead of using lock-nuts (hate those things) use a regular nut with a lock washer. You will be much happier if you ever have to remove your manifold again.
Rogue