If Apple goes the sub-notebook route, they should try to nail the tablet market. No one's come out with the 'killer app' in that realm and everyone I know would love to have one. Students who'd like to carry a MacTab to class and take handwritten notes directly into iWork, raise your hand. Sooner or later the iPod bubble will burst, innovate some more.
I could see Apple altering the MacBook lineup a little bit, in two ways:
differentiating the BlackBook further from its fellows (to justify the surcharge) - optional matte screen (PLEASE), discrete graphics, 1 512MB SO-DIMM stock.
or
offering the the black throughout the MB line, and simply offering a top-shelf MB with those features.