I have the same problem: iPhone 4, 3,5 months old, never in water, never got a spill of liquid over it. I had it in my pocket and was very carefully with it since I hesitated a long time to buy such an expensive device. Most extreme 'water' exposure might be that it was in my pocket in a bathroom. But it never got actually wet.
The guy of the store noticed a red detector near the connector (the one in the phone plug was white) and said it got wet, the warranty is gone and it will cost me 200 euros. I feel utterly betrayed and can't believe this is happening. I am charged of being a fraud and supposed to prove my innocence.
I am using Macs since 1988, always had good service on my computers. As others said: this is just faulty design. I have old Nokia and Samsung phones at home, they have plastic covers over their jackets. That is apperently what is missing here. If Apple can put in these detectors, they surely can design a flashy cover??