A few months ago Apple asked the US copyright office to rule on whether jailbreaking the iPhone violated the copyright legislation. The latest event in the saga is a recommendation by the US Library of Congress on the DMCA. You can see the relevant information in this excellent article by Will from interMobile.
What the DMCA act says, and also the Australian legislation as amended in 2007, is that it is a violation of copyright to break encryption, or to write software that breaks encryption, or to distribute software that breaks encryption that is in place to prevent copyright software from being copied or distributed in violation of the terms of the copyright.
In practice this means that ripping DVDs and Blu Rays by breaking the encryption is a violation of the copyright. Even writing and selling the software is not permitted. The Australian legislation does have several exemptions. My reading of it suggests that non-commercial use is permitted so long as the copyright of the work in question is not violated. In other words it is OK to rip CDs, DVDs and BluRays for personal use or if the work itself is out of copyright or in the public domain, or the work permits copying, or for "fair use".
However jailbreaking an iPhone does not in and off itself result in the ability to copy protected software to other devices. A further step is required of actually breaking the encryption on the app itself. For this reason jailbreaking does not violate the DMCA according to the Library of Congress. To establish this as law it needs to be tested in court but this is unlikely since this recomendation from the Library of Congress makes it unlikely that such a case would succeed. I think it is clear that Apple has accepted this as an umpires ruling. This will not stop Apple from trying to prevent jailbreaking though.
As far as Australian law goes we can safely assume that jailbreaking is not a violation of the Australian copyright legislation. In my opinion neither does it affect hardware warranty. We have very clear rights in law against hardware defects and despite what any one says nothing that Apple says either in their conditions or in public statements can negate our statutory rights. In any case you can easily do a restore before taking the phone back for repair/replacement.
Hopefully this will put an end to the ill-informed statements made by some people regarding the legality or otherwise of jailbreaking.