Woah, are you saying that you can root a google nexus phone without voiding the warranty? I assumed you weren't allowed to do that, would be great if you can.

Yeah, you can. And at least on the newer Nexus phones, you can even re-lock them without any trace of it ever being unlocked.

yup. you just reflash the stock rom, then relock the bootloader, and it's like new

It's my impression you might get a warranty claim denied on a Nexus device if your problem is software-related and you have modified software on the phone that could be at fault. I had some instability and camera trouble due to a custom kernel, for example.

The Nexus 5 bootloader unlock confirmation screen says it may void the warranty. From what I've heard, LG/Google never gives people trouble about root when there's a hardware problem.

The catch is that the device gets wiped during the unlock, so someone can't disable a PIN-lock by going through the back door and access sensitive data.