I think there are at least two perspectives on the question. The perspective Steve addressed is "does what you wear impact your performance". I believe Steve is correct that it doesn't. I've been just as productive in a suite and tie (banking work) as I have in a bath robe (telecommuting). It's probably a valid point that that the more comfortable you are, the less outside distraction there is and therefore; the more productive you are. Another perspective is "what you're trying to communicate with your image". In this, other peoples perception of your competence and professionalism is their reality.

Me personally, I tend to dress like my boss. If my boss is on an upward track (she is) then that position will become open and it gives upper management a "warm fuzzy feeling" that they won't need to modify my behavior to have me fit that role. Some will say that's silly however; being dressed and behave in a manner that the company isn't embarrassed to stick me in front of a client hasn't hurt my career. It all comes down to what you want to accomplish.