http://www.apple.com/support/iphone/service/faq/#faq26
What kind of activities will void my warranty?
Apple’s Limited Warranty for iPhone excludes coverage for damage resulting from accident, unauthorized service and unauthorized modifications. Please review the warranty for further details.
Apple reserves the right to refuse service on iPhones that are outside their warranty period if damage has occurred due to accident, abuse, or misuse. In such event, Apple will return your original iPhone to you without performing service, and may hold you responsible for diagnostic fees. See
Apple’s Repair Terms and Conditions for details.