I work in research and until last week I had always washed my Band pretty thoroughly when I wash my hands before going home. Last week I had to reach into a pretty gross pool of water with my Band hand so i washed the Band extra vigorously. At some point during that, water must've gotten into my Band and an hour later, the LCD was nonfunctional. I'm thinking it was probably my washing rather than the pool that did this.
Anyway, being that my Band was only 5 months old, I took it to the MS store two hours away and they replaced it with a brand new one without asking any questions whatsoever. From other forum posts I've seen, going to a physical store yields a pretty good chance that they'll replace it, while calling the support line gives mixed results.
Moral to the story, if your Band is less than a year old and you live close to a MS Store, take your Band there if it stops working.
Also, I don't want to hijack this thread but my new band seems slightly different than the first one. The rubber on the strap or something just seems different. Maybe they've discretely improved some materials or maybe I'm imagining things. Anyone else notice this?