In the US, at least, the legality of jailbreaking currently rests on a temporary DMCA exception that needs to be renewed every three years. Prospects for the renewal look good at the moment:

http://www.teleread.com/copy-right/jailbreaking-dmca-exempti...

But that is no reason to be complacent about the issue (or to think that this legality is written in anything but sand).