Just about every mobile phone I have used has a moisture indicator somewhere in the battery compartment so a rain drop here and there is not going to void your warranty. I have never dropped a device in water and have never seen this indicator activated on a device before. If you have an iPhone though, you better take extra care to keep any water or moisture from getting into the headphone jack opening where the indicator is located. Since the iPhone has no removable battery or battery compartment Apple decided to place the indicator in a poor location that is actually quite easy to get moisture into. If you use your iPhone to work out you could easily have sweat drip into the opening or if you walk in the rain a drop could easily slide down your headphone cable and into the opening. CNN reported that people are now discovering how sensitive the iPhone can be to moisture.
This looks like it could be an issue for any iPhone or iPod that uses this type of indicator system and even if you have your device in a case you may still get moisture build up in certain environments. I think having Bluetooth A2DP support (coming in iPhone 3.0) will help with this issue and I personally use the elliptical at home with a Nokia E71 or Nokia 5800 connected to a Nokia Bluetooth A2DP headset to avoid cables flying around and the issue of sweat on my phones. I am bit surprised that Apple hasn't led the way with iPods and Bluetooth connectivity for stereo sound. The Nokia devices and headset I use NEVER skips a beat and the sound is quite good. Granted it isn't as good as a direct wired connection, but at least my phone isn't getting all wet from sweat.
Have you found a case that could prevent this issue from occuring while you work out? Have you ever had this issue affect your iPhone or iPod?