A while back, Leann Ogasawara, Canonical Kernel Team Manager, has said
that it has been discussed internally to use a rolling release model for Ubuntu between LTS releases, but that it is just an idea for now. With the new Ubuntu Touch
, which needs both "velocity and agility", the rolling release mode seems to be more than just an idea and Rick Spencer, Engineering Director at Canonical, has made a proposal about this on the Ubuntu Devel mailing list
In the thread named "Let's Discuss Interim Releases (and a Rolling Release)", he states that "we should keep LTS releases, but starting now, stop doing interim releases and start a rolling release".