Every three years, the DMCA or Digital Millennium Copyright Act gets reviewed and laws are added and subtracted. This years review process seems to be going in the right direction as congress has added five new exemptions to the DMCA this year that might allow the breaking of DRM legal if used for fair use rights.
Jailbreaking cell phones is really where all this started but has since lead to legalizing the breaking of DRM on video games, e-books, and DVDs as well as bypass external security measures on some computers. If you paid for the device, you should be able to use it how you see fit.
Please don't forget though, this doesn't make piracy legal.