I was thinking the exact same thing this afternoon. I cant believe Apple would phase out the macbook, but the little plastic guys design is getting pretty long in the tooth. Apple have been pretty good at keeping the plastic macbooks internals up to date with their unibody counterpart though, even for a few days providing a 'better deal' than the aluminium 13" before it got its sticker bump. So its not got the same fate as the .... venerable eMac.
But where do they go with the Macbook? keep it plastic? that would seem to jarr with their current design aesthetic of "glass and aluminium" and all those eco-warriors on apples back. a macbook suggests a folding screen, so i doubt this particular product would become a tablet. So a netbook? Perhaps, but if they were heading that way, wouldnt they want to ....
make it smaller?
make it more svelte (thinner)?
remove all that superfluous gubbins like DVD drives?
That sounds an awful lot like a Macbook Air to me. My humble opinion is that the Macbook and Macbook air might merge into a single product, branded simply as a "Macbook". The MBA can certainly lose an inch or two around the bezel, and the Macbook cant stay in its little plastic shell forever.
Sure the Macbooks future could be a tiny netbook and the Macbook air could stay as a full sized 13" flat laptop, but i think it would be much more elegant for these two products to meet at a middle ground.