jailbreaking _phones_ is _possibly temporarily_ legal, as an exception to DMCA by the Librarian of Congress which has to be explicitly renewed every few years.

Jailbreaking tablets is not legal.

Providing or distributing software intended to help you jailbreak is also illegal, even though jailbreaking may be legal.

See the EFF on this (and donate some money to them!): https://www.eff.org/deeplinks/2012/11/2012-dmca-rulemaking-w...