Even developer phones such as the Nexus One lose warranty the second you unlock the bootloader.
Strictly speaking, in the US at least, you don't. Oh, they might SAY you do. They might even act as if you do if you try to do a warranty replacement through normal channels. But if you really pushed the issue and filed a lawsuit against them (or got together with other Nexus One owners to launch a class action lawsuit), it would be almost an open and shut case in your favor thanks to the Magnuson-Moss Act, which was passed around 35 years ago.
In plain English, MMA basically says that a manufacturer can't arbitrarily deny warranty coverage unless they can prove that whatever it was you did literally and directly caused the product to break. At worst, it means HTC has the right to refuse warranty service for your phone unless you agree to let them wipe and reflash it to the stock factory rom first, and give it back to you with that stock rom.
It was passed because in the 60s and early 70s, American automakers tried invalidating warranties if you installed an aftermarket stereo, used "unauthorized" replacement parts, or basically did anything to it yourself besides add gas. Pre-MMA, Ford could refuse to repaint a peeling bumper because you installed replacement bulbs in the headlights that weren't sold by Ford.
There's a bit of legal gray area over whether or not HTC could legally charge a fee to Nexus One owners for reflashing their phones to stock. IANAL, but I believe HTC would lose unless it could demonstrate that the end user COULD have reflashed it to stock himself, but didn't.
As a practical matter, just about the only thing HTC (or any other handset maker) could really claim and have a chance in hell of prevailing over is if the phone's flash rom failed due to excessive erase-rewrite cycles that were FAR in excess of what any reasonable user might have expected to survive (ie, if they could prove that you flashed a kernel with a bug that madly wrote and rewrote a chunk of flash and wore it out prematurely).
Remember, at the end of the day, Verizon doesn't own your phone... you do. You purchased it, and you own it. It's yours. In fact, Verizon
can't force you to lease it from them, because they're subject to the same consent decree that forced the breakup of AT&T 25 years ago. One of its terms was that they MUST allow customers to purchase their own phones, as long as they're physically compatible with the network. AT&T is subject to the same consent decree. Sprint & T-Mobile are a gray area... they're not descendants of AT&T and aren't subject directly to the decree itself, but they ARE subject to FCC regulation... and it's likely that at some point along the way, the FCC incorporated those terms into its own regulations.
Ownership of your phone is important, because it gives a STRONG legal claim to anyone whose phone gets bricked by Motorola's bootloader protection. American law (and European law) frowns mightily upon self-help remedies that attempt to enforce arbitrary contract terms by vandalizing hardware that doesn't belong to the vandalizer. DirecTV can fry your DVR if you tamper with it, because they legally own it. Verizon doesn't own your phone, and legally, they can't force you to let them own your phone.
Magnuson-Moss is *so* well-established in the US, companies don't even dare to *try* challenging it, because it would be legally suicidal. Two years ago, MSI shipped netbooks with a gig of ram, an empty soDIMM slot, and a sticker over the case screw stating that its removal (to upgrade the ram) would invalidate the warranty. Less than 2 months after the netbook hit stores, MSI had a statement on their website apologizing for the "confusion" clarifying that merely puncturing the tape, removal of the screw, or opening up the case for the specific purpose of ram expansion would NOT by themselves invalidate the warranty. I believe Britain has a similar law, and got a similar press release from MSI of its own a week or two later.
Getting back to Motorola... by just about any American legal standard, Motorola would lose if you as a consumer tried to reflash your phone, and Motorola's bootloader intentionally bricked it. HOWEVER, if Motorola's bootloader reacted to an attempt to "tamper" with the phone by wiping all of their copyrighted files from its flash, and left you with a phone that was useless at that point as a phone, but HAD a functioning bootloader and could (in theory, if not reality) be used to reflash whatever you want onto it (AOSP or whatever), they'd be safe. If the phone had a protected block of flash holding an encrypted backup copy of Motorola's ROM, and booting with a modified copy produced a message asking for permission to rewrite it back to stock, wipe the filesystem and flash your own image from the uSD card, or shut down, they'd be on about the safest legal ground you can be. Where their current policy crosses the line is bricking the phone beyond consumer repair in response to an activity that's entirely legal for a consumer to do with a phone he has purchased.