I am a sweaty monster.
I had a lovely Specialized "Mountain Man" helmet a few years back. I watched that foam "melt away" from the inside over the years. It was creepy. Like watching a snowbank melt in a warm breeze.
One day (about 4 years in) a chunk of foam crumbled and fell out.
I switch helmets every 3 years now.
Mind you, my sweat is amazingly caustic. I melt the barrels of some pens, my sweat has dyed my iBook keyboard bright lime green, and any perfume completely changes it's smell once it gets on my skin.
My favorite helmets are generally the cheaper ones, anyway. I don't mind the expense of replacing them, even if my old helmet doesn't look as bad as that Specialized.
(but I kind of figure the cosmetic damage I can see is probably just a hint at the vital structural damage I can't see.)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson