I define "geek" by passion, not expertise. You're an geek if you do a lot of spare-time reading about the subject, if you'll neep about it in person or online, if you'll listen happily to people who know more than you do, if you have Strong Opinions.
I am a geek about:
New Who-verse (Who, Torchwood, Sarah Jane Adventures)
Life On Mars
Pop music, both British and American, but only through the 80's.
Pop culture in general, both British and American.
In the past I have geeked about:
Star Trek-Original series
Things I might geek about if I had more time/energy, that is, stuff I'm enthusiastic about but not sufficiently well read or practiced in:
What are you a geek about?