DMCA's revision deemed "jailbreaking and unlocking" of smartphone legal, but Apple says that "jailbreaking your iPhone, while now as legal as can be, will still void your warranty." An Apple spokesperson responded to DMCA's Jailbreak Exemption said:
"Apple's goal has always been to insure that our customers have a great experience with their iPhone and we know that jailbreaking can severely degrade the experience. As we've said before, the vast majority of customers do not jailbreak their iPhones as this can violate the warranty and can cause the iPhone to become unstable and not work reliably."
For more information about Apple's stance on jailbreaking, see this support document: Unauthorized modification of iOS has been a major source of instability, disruption of services, and other issues.