This initiative has been in the works for a couple of years, but this afternoon the Register of Copyrights finally granted explicit exemption status to the act of jailbreaking your mobile computing device, to include those running iOS.
In short, you cannot face legal action for jailbreaking your device. Prior to this ruling, it was ambiguous whether the act of jailbreaking constituted a violation of the Digital Millennium Copyright Act.
Ars Technica & Engadget have good summaries on the impact of this exemption:
For more information on the concept of jailbreaking, this Wikipedia entry provides a good overview:
Additionally, if you're interested in the larger scope of mobile computing ethics, here is a paper on the topic I authored last year: