Uhhh...
Isn't this already in the current iPhone? it's a little plate that changes colour inside the headphone jack....
http://www.iphonebuzz.com/iphoneipod-touch-have-water-seals-on-them-171643.php
If you’ve dropped your iPhone down the privy and tried to get a new one by saying “it simply stopped working”, your cover’s blown. Apple is working to prove when a gadget’s been damaged by water. apple_water_damage_patent_app Apple's patent app could see an end to false warranty claims According to an Apple patent …
Most phones do this already.
Sony Ericsson and Samsung phones have it on the battery, with the SE ones also inside the phone in a visable place. Some Nokia handsets have them visable, and most motorola phone hsav ethe under the battery as in the apple "pattent". Most start life out as white, but then turn red when they have become wet.
Also, most water damage, at least on phone, will show up with corrosion on the PCB, an usually blue/green corrosion on any copper contacts.