I just have to restate my disagreement with the OP.
It really is just a matter of time. Next September you'll see more unibodys, simply because there are more new laptop buyers and the people with the plastics either graduate, upgrade or the machine breaks down.
The difference between the plastic and the base unibody is $250 with student pricing. You get 40Gb more, double the ram, better build quality, a newer processor, and a graphic card that doesn't absolutely suck.
Even as a student who only works during the summer, I would easily decide to go with the unibody. Some of my classes have textbooks that cost more than $250.
Everyone that I know that's buying new laptops and is interested in Macs is doing the same thing. Oh, and we're not all preppy daddy will pay for it types. Some have their parents pay for it, some spring for it themselves by working an extra shift or buying less energy drinks/alcohol and some do really stupid financial decisions.