Then go back to the store and talk to the manager. Politely. It usually takes more than one moisture sensor to void the warranty. If the phone had never been opened or altered, and if there was only one sensor tripped, then normally, I'd think they would replace it under warranty.
Unless, of course, you told them that it was working just fine until you took it out in the rain and it might have gotten a drop of water in it, which of course wasn't your fault, but that as soon as that happened, it didn't work any more...
If you told them that it got wet right before it stopped working... well... you can fill in the rest.
What, exactly is the problem that prompted you to take it in for service in the first place?
Apple has not made public a policy regarding the water sensors. The only statement available to the general public is the warranty statement:
which states that the warranty does not apply to "to damage caused by accident, abuse, misuse, liquid contact, fire, earthquake or other external cause"
If the technician at the Apple Store has told you that the warranty has been voided, then there's probably nothing you can do. You can try discussing the situation with the store manager, but they usually go by the technician's diagnosis.
Many Genius are now trained to be much more sophistocated and will void the Warranty if they dectect corrosion, even if all the Liquid Indicators are perfect, unexposed (white). The Genius makes the determination, no matter if there is 0, 1 or more indicators tripped. If you are not satisfied you can spend your time trying another Genius or sending iPhone to Apple for evaluation.
really appreciate your replys guys, maybe i'll have a chat with the manager, i originally took it in because it wont charge, now that could be a dead battery!!! anyway i've just been browsing this issue online and i'm shocked to learn this is a HUGE worldwide issue with careful owners having warranties being voided because of these sensors that can trip in many circumstances that do not damage the phone in any way.,
Anyway for me the name of Apple has been tarnished and i'll move on but as Apple are so mainstream these days losing loyal customers isn't really going to hurt....which is a shame