A while back, Ranil Illesinghe has a nice post on the difference between individualism and non-conformism. The whole entry is worth reading, particularly for people new to Objectivism. I particularly liked the litmus test at the end:

I find that the best test to see whether or not someone is an individual is to ask the following question: If the mainstream acted in the same way that he acted, would he:
A. Stay the same OR
B. Do things differently?

If he chooses to stay the same, then he is a true individualist, because it does not matter how many people act LIKE you, as long as you are relying on nothing but your independent judgment. If they start being different for the sake of being different, then that person is the worst sort of coward — the fashionable non-conformist.

Can you imagine Howard Roark abandoning his style of buildings simply because great masses of people started recognizing their superiority over the buildings designed by Peter Keating or Gus Webb? Perish the thought!