I can't see that happening. They aren't going to insure a device that is rooted since someone could get it.. Root it.. Then overclock and fry it. Completely the users fault.
Frying your phone from overclocking is very unlikely, since your phone still enforces thermal limits that will cause it to shut down. Brciking, on the other hand, is all too common. Sadly, that's basically the one time they can't check for root prior to accepting it under warranty. I've known people on other forums to mess something up using root, then intentionally hard-brick their phone so that the service center can't turn it on to find out it was rooted.
The people who get screwed are the ones who root safely. Something like a dead pixel is obviously not caused by root, but is instead a manufacturing flaw. But HTC and Samsung put a flag to show that your device was rooted, so even if I am able to restore my phone fully to stock they'll still be able to tell. That's a major reason I'm sticking to Nexus devices, since Google actually recognizes that it's obviously not a problem caused by root if you are able to restore your phone to the factory image.