This, of course, is how an iPhone tells you it is water damaged. The warning message will appear intermittently even though there's no attached accessory.
My son's SIMless iPhone has done this for two years. That's not a surprise, it was a gift from a friend after a white bowl swim.
My iPhone 4 though, it's never been swimming. It has, however, spent a summer in Bangkok CO2-enhanced Minneapolis. It's had to live with sweaty pockets and big swings in temperature and humidity. It's acting like a wet phone, even if that is from ambient moisture.
I turned it off for a while, and a cooler drier time of day it seems well again. It's out of warranty, so I'll wait and see how it goes. (I did use a sturdy wooden toothpick to remove some rock-like lint from around the base connector. )
This is annoying. Multiply this annoyance across a family of five and annoyance becomes a serious problem. The iPhone is just too damned "precious".
I doubt iPhone 5 will be any better. If there's hope, it's for iPhone 6. If Apple wanted to, they could make that phone a much less tender flower.
Why should they want to though? After all, the more tender the iPhone, the more are sold. Until, of course, the brand gets such a reputation for fragility competitors begin to run ads showing "this accessory is not optimized for this iPhone". (Hint, hint).
There's reason for hope. Apple is very good at taking just enough blood -- but not too much. After years of fragile laptops they did move to the more robust uniblock aluminum body.
iPhone 6, please be less precious. In the meantime iPhone competitors, stop talking about apps and start talking about moisture resistance.
 Anyone know of an non-builky iPhone case that seals the dock connector? I am consider the OtterBox Commuter.
Update 8/4/11: I wrote a tech post.on wet iphone prevention and treatment.