While Apple would deny warranty service if they discovered the phone is or was jailbroken, a full restore from iTunes completely reverses a jailbreak, so as long as your phone is either dead or restorable, there are no warranty issues.
When I look at the mobile phone market, I am quite willing to use a G1 solely to support the prospect of a handset that I can control. The more people use an iPhone because it is epsilon better, the less companies will be inclined to give users freedom.
That didn't fix it? Ok, reinstall windows and the latest drivers from our website.
That didn't fix it? Ok, here's an RMA.
Not that different from other platform vendors...