Apple's notebook lineup has received a significant revamp over the past two years, and the introduction of the MacBook Air seems to have finally edged out the low-end MacBook design. While originally priced as a premium product, the Air seems to have found a more mainstream market as its starting price dropped to only $999.
Many now expect that Apple's design choices in the Air will eventually make their way to the MacBook Pro product, with the use of integrated SSD and lack of optical drive being the most notable changes allowing for such a thin design. While we don't know for a fact, we expect that any future "ultra thin" laptop from Apple will also dispense with a built-in optical drive. As evidenced by its recent release of the optical drive-less Mac mini, Apple has no problem leaving physical media behind.
The ultra-thin market is also about to get much more competitive this fall as Intel's partners begin launching their Ultrabook notebooks.
The timing of an ultra-thin 15" Apple notebook remains a mystery to us as Apple just revamped the MacBook Air with new 11" and 13" models. Meanwhile the MacBook Pro line was refreshed in February, likely pushing the next release out to at least very late in the year.
Update: TUAW corroborates our claim and also adds that they believe that this new notebook will fall under the "MacBook Pro" branding alongside a thin 17" model. Also, they believe it might be available in time for Christmas.