Do you dress up for work? Does your office have an unspoken dress code, pressuring people to dress up or down?
On the other hand - I do like to wear "better" clothes. They definitely do work in places like banks, offices, etc. where you want to get something done efficiently. They will even get you a drink at the bar before a student in a hoodie sometimes. There's a stereotype of a professional and expensive look - play it right and you can get a slightly better treatment.
Additionally if you do feel that you make a better impression than people around you, your charisma will improve making the "better clothes" a kind of self fulfilling prophecy. This could work just as well for having the most geeky tshirt in an appropriate group.
The TV show "What Not To Wear" has some dramatic makeovers that greatly improve people's self-confidence.
I seem to remember an HN story about a fashion blog for geeks, but I can't find it now. If someone were to create one, a good name might be "DressCode". :)