As far as merging, It seems to me they already tried that with the aluminum Macbook. I'm hoping that what they might do is offer a 11.6" white Macbook, with the main differences with the 13" being a lack of optical drive and the obviously smaller screen. All the ports the Macbook now has should be retained. The thing about a 11.6" macbook is, it would be VERY popular with kids that would otherwise end up with a netbook. With a price around that of a high-end iPad with 3G, say $799 or so, it would not be in the same market.
That said, the main thing I've noticed people like about the new Air's is the instant on that the SSD provides. Apples event implies, as does history, that what's brought into the MBA propagates to the other Mac computers. My question is, can instant on be delivered to the rest of the Mac line without resorting to proprietary Storage? Right now my Macbook can be closed, then opened and it seems to be able to start where I left off, but if I unplug it the "standby" time isn't 30 days.