[Nexus 4 Q&A, Help & Troubleshooting] jasonwc:
Magnuson–Moss Warranty Act

For US consumers, it's important to note that manufacturer's warranties are governed by the Magnuson–Moss Warranty Act. In this case, unlocking the boot loader MAY void your warranty in certain circumstances, but only if the defect is proximately caused by the action that voided the warranty.

For example, if you unlock your bootloader, and later flash the wrong radio, resulting in a hard bricked phone, the manufacturer can deny your claim. In this instance, the defect was caused by the consumer, and it would have been prevented had the phone been left in its default locked state with stock Android.

In contrast, if you unlock the bootloader, and flash a custom ROM, you will still receive warranty coverage for a defective screen or broken power button. In this instance, there is no causative relationship between the defect and the consumer's action. And, I've been told by several reputable sources that if you seek warranty coverage for a hardware defect like a screen issue, no attempt is even made to verify whether the phone has been rooted/had its bootloader unlocked.

However, companies still write overly broad warranty language, and sometimes attempt to enforce such language, even where it would violate federal law. This is often a result of employee ignorance rather than corporate policy. For example, I purchased a Netgear MOCA adapter from Amazon. The device was manufactured in 2009, but I made the purchase in 2011, and the warranty policy was 1 year from date of purchase. One of the devices was defective, and I made a claim within a month of my initial purchase, but was told the warranty is actually from the date of manufacture, and had expired. I read the employee the terms of the warrranty, and explained that I was prepared to sue Netgear, if necessary. Within 5 minutes, I had an RMA.

Know your rights, and insist on speaking with a manager. If that doesn't work, send a letter to the executives at the company in question, and CC your congressman, senator, and the Bureau of Consumer Protection at the FTC. I ended up having to do this when Verizon chaged me a $300 ETF for a Verizon FiOS contract. I was told when I signed the contract that I could cancel without an ETF should I move to a location that did not have FiOS availability. I did in fact move a year later to a home without FiOS, and they charged me an ETF anyway. After numerous unsuccessful attempts with FioS customer service, I made a complaint with the FCC, and forwarded the complaint via email to Verizon's CEO, my senator, and congressman, citing to the complaint #. 24 hours later I received a call from Verizon's executive customer support team. They waived all fees and apologized profusely. I ended paying $0.

Note that companies MUST respond to FTC Consumer Bureau complaints in some fashion, and they can face fines and other negative repercussions for failing to adequately resolve consumer complaints. Generally, the employees at this level have far greater authority to resolve your specific issue than a CSR.

If you are dealing with a large entity, consider filing a Better Business Bureau (BBB) complaint. Companies seek BBB certification, and have dedicated employees that resolve such complaints. These employees often have far more authority than regular CSRs. In addition, many companies will go out of their way to resolve these complaints as they are publicly posted on BBB's website, and are used to grade the company by the BBB.

I AM a lawyer, but this should not be construed as legal advice. Just some helpful tips from a consumer that has had to deal with my fair share of crappy situations.