It's been a point of contention virtually since the iPhone shipped, and especially since hacking the device (by hacking, in this sense, we mean the addition of third-party native applications via the use of Installer.app or other means) became a mainstream practice: Will the addition of such software and/or modification of the iPhone's default software set result in the warranty being voided? The answer, according to Apple technical support representatives we contact is: yes, in a way.
Here's the deal: Apple representatives told us that adding third-party native software to the iPhone does indeed result in the warranty being voided. In Apple's one year limited warranty PDF document, the company states that the iPhone's warranty does does not apply to:
"damage caused by accident, abuse, misuse, flood, fire, earthquake or other external causes; (c) to damage caused by operating the product outside the permitted or intended uses described by Apple; (d) to damage caused by service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (Ã¢??AASPÃ¢??); (e) to a product or part that has been modified to alter functionality or capability without the written permission of Apple; [...]"
According to the Apple reps we spoke with, the addition of third-party applications will fall under the "accidental damage" clause, and hence phones that are brought in for service with evidence of third-party software modification may be denied service, and potentially have their warranties permanently voided, meaning that future service will not be delivered.
However, the Apple rep we spoke with said that -- though this is information he is generally "not supposed to share" this common sense fact with customers -- iPhones that have been "hacked" (received third-party software modifications) then restored to an original factory state will certainly be eligible for service. In other words, if Apple can't prove that you've made third-party software modifications to the iPhone, it will have no grounds to terminate the warranty; a fairly obvious fact.
The moral of the story: Restore your iPhone before bringing it in for service. You can do so by connecting it to your computer, then in iTunes, selecting your iPhone and click the Restore button under the Summary tab.