I agree with the points Misty made.
If you manage to get the stains polished out on the top, you might consider doing what a friend of mine in the same situation did to prevent it from happening again: he put a John Pearse armrest on the guitar
Here's a photo of the Pearse armrests on some of Roman's guitars, from a thread on the Taylor Guitar Forum:
I don't have sweat stain problems on my guitars, since I'm Cool Hand Luke (or, maybe, "Cool Forearm Wade,") and don't sweat on them. But I like the armrests for the way they lift the forearm off and allow the top to vibrate to its fullest extent, thus allowing the guitar to achieve its richest possible tone.
Hope this helps.
Wade Hampton Miller