According to Canonical’s Kernel Team Manager, Leann Ogasawara, it is possible that Ubuntu will get rid of the current “new release every six months” model and move to a rolling release. (You can find more info in this recent video.)
So, just what is a “rolling release”?
It’s exactly what it sounds like, really. As individual new/updated packages are ready, they are put up on a repository and made available to everyone. New version of Firefox? No need to wait until the next big release of your operating system…you get it right away. New improvements to the Desktop Environment (such as Ubuntu’s Unity)? BAM! No waiting until next April. Immediately available.