I'm on my second iPod shuffle. The first one wouldn't play, and I returned it, and was told that I shouldn't sweat on it. I was a bit irate, as all their advertising shows an attractive women on the crosstrainer, shuffle around her neck, and I would assume sweating.
Anyway, bought a replacement, and also purchased a clear sweat-proof cover at Target. So far (4 months later), it's still working well.