I don't need to differentiate my MBA because nearly everyone else on campus has a MBP (if they do have a Mac). In fact, looking out in the lecture room right now (a small class of 15 people), I see one white plastic MacBook, five MacBook Pros, and one MacBook Air (not counting mine). Oh, and three Windows laptops.
If I had to choose something to make my MBA different, I would probably opt to get a design laser etched into the lid. Not sure yet what I'd pick though.