When I had my iPhone 3G replaced under warranty (no problem with the moisture sensor BTW), I spent some time talking with the Genius about iPhone reliability. He said the #1 reason they get iPhones back for repair is due to moisture damage. They see the headphone moisture sensor is "tripped" and they almost immediately decide it's the customers fault. He said there have been cases where they could tell when the customer was genuinely surprised about the sensor being tripped and in some cases they HAVE replaced phones under warranty, even with tripped sensors. But he said those cases are rare. He further said they sometimes open up the iPhones in the back and, when they do, 100% of the time the internal moisture sensors are tripped too.
In other words, in HIS experience, when they open up someones iPhone that insists it was never near water, 100% of the time that person was either lying or didn't know someone else dunked their iPhone in water (or whatever).
He also said a fair percentage of people come in admitting the iPhone got dunked in water (or whatever). The most common is falling in the sink, bathtub or toilet. The most interesting one he had so far was someone dropping a guy's iPhone into his mug of beer at a bar. The guy apparently left it on the counter when he went to the restroom and, when he came back, it was in his beer.
Mark