We should stop conflating the term 'geek' with the term 'nerd'. I think most people familiar with the cultures in question would agree that there is an increasingly clear difference between the two (although membership in the two is certainly correlated.)
I don't think there's a concrete, universally accepted difference. Pretty much everyone has their own definition of "geek" and "nerd," and how much they overlap, and most of those definitions are mutually incompatible.
People can have different, mutually incompatible definitions of the two, that's fine. All I'm saying is that very few people who have any opinion on the subject think the two refer to the same set of values and behaviors.
Do they have importantly distinct definitions? A quick check via Google implies they're both about "unfashionable or socially inept person" or "foolish or contemptible person who lacks social skills or is boringly studious" but that's not necessarily how I view either term.
I always figured that "nerd" implied some sort of technical knowledge or scientific mindedness. Hence slashdot being "news for nerds, stuff that matters". Geek then I suppose, would be your standard comic book store customer.
Enough people use them interchangeably that it seems there is no longer any meaning. We should probably just jettison both words.
I don't know that I've observed people using the terms interchangeably, and the distinction between geeks and nerds is pretty constant - what does seem to happen is that people disagree about which label ("geek" or "nerd") goes with which category.
