If you watched the ad for the new MacBook Air, you will note that Apple didn't introduce it as the new MacBook Air; they introduced it as the next generation of MacBooks.
http://www.apple.com/macbookair/#video-ad
It's pretty obvious that the next revision of MacBook Pros will be sans the traditional hard drive and optical drive. They will be thinner and lighter. Will Apple keep or discontinue the 13" MBP is then a very reasonable question.
I think the way that Apple will distinguish the Pros from the Airs will be faster processors, more ram, more ports for greater connectivity, and larger hard drives by default with higher BTO options. But, I think the biggest way they will distinguish them will be higher battery life. It's a different kind of portability. If you have a lighter and thinner MBP that you can take with you (without having the carry the power adapter) and work in Adobe CS for 10 hours with confidence, not having to wonder if there is an open outlet at the Starbucks, you have a different kind of portability and greater productivity.
The biggest question remains: what are they going to do with the processors? The sun sets on C2D next year. I doubt Apple stockpiled 400,000 C2D chips to get them through another year of MBA deliveries, so does the MBA get a quiet update with Sandy Bridge in early 2011? If so, the better battery life of the 13" MBP may not be enough to distinguish it from the MBA and justify its existence.
The white MacBook is definitely due for EOL. If you want an entry level Mac at that $999 price point, the entry level 11" MBA is the clear choice.