This rumor has been around for a while, but there's some fresh news on it today. Upstream suppliers of Apple have reportedly started shipping a small volume of components for a 15-inch ultra-thin notebook model from Apple in November, and the device could be one of two things.
First, a MacBook Air, but 15-inch, or a thinner MacBook Pro. The new 15-inch ultra-thin MacBook is expected to arrive in Q2 of 2012 according to sources from the upstream supply chain. Mass shipments of the new 15-inch ultra-thin device are expected in March and could be catalogued in either the MacBook Air or MacBook Pro line.
This could be the competition to the Ultrabook line recently launched by companies such as ASUS and Acer. We'll have to wait and see.