[jyl]

The iPhone isn't advertised as water proof or even water resistant. And salt water (sweat) is especially bad for electronics. If you sweat so much that you leave your clothes soaking and puddles on the gym equipment, then better use a good iPhone case.

FYI the phone has some moisture sensors inside the case - at the headphone jack, dock connector, and two other locations. Reportedly, if any of those moisture sensors show that water has entered the phone, Apple considers the warranty void.