While I entertain the idea of a 15" MacBook Air (given that I don't feel like ultra-portable and large screen should be mutually exclusive features for those that want it [a demographic that I'm not a part of, mind you]), the idea that such a machine will completely replace the 15" MacBook Pro as we know it today is absurd and demonstrates a clear lack of the understanding of these machines under the hood.
I know that (seemingly) all of you are hell-bent on the death of the optical drive in favor of narrow and nowhere-near-all-encompasing replacements such as iTunes (what about my movie collection on discs?), the Mac App Store (what about my Microsoft Office and Adobe products, or my pre-MAS software?), or cloud-based storage things (can't easily hand off 4.7-8.5GB of data to a friend via dropbox or iCloud!), but really, the lack of an optical drive is only one of many inconveniences to owning a MacBook Air that will NOT appeal to EVERYONE wishing to own an Apple notebook computer.
For one, removable/replacable RAM. If you buy a MacBook Air today, you're capped at the RAM you have; for some, this isn't a big deal; for others, it's a deal-breaker. Not to mention that you are guaranteeing that when Mac OS X 10.8 comes out and it requires a minimum of 4GB of RAM, that MacBook Air you bought with only 2GB of RAM only two years prior, won't make the cut. Similarly, if you buy one of today's Airs, and the trend of OS X's RAM requirement doubling at every release continues, it won't be able to run 10.9. I'm sorry, if I'm buying a Mac today, it better be able to run 10.9. Add to that, non-removable (at least not without that proprietary pentalobe screwdriver) boot drive, a non-standard boot drive form-factor, lack of on-board expansion (and sure, for a substantial cost, I can regain that expansion back with a Thunderbolt device), you have a notebook that is pretty limited.
Of course, I haven't even gotten to the part where you won't be able to fit in a discrete GPU (due to the much smaller thermal envelope) or the quad-core processors, let alone non-ultra-mobile (and therefore not-severely-underclocked) processors. If I wanted an iPad, I'd buy an iPad. But a MacBook Pro still affords options that you can't (and at this rate, never will) get on a MacBook Air, and for that reason, with Apple's notebook sales being the most important of their Mac unit as a whole, I don't expect the MacBook Pro to go anywhere, whether this 15" machine sees the light of day or not, and neither should you.