@Trasher
the one thing to look for in a fixed gear or single speed bike/frame is that the rear wheel mount faces straight to the back. it's designed that way to allow for tension in the chain. high tension -> higher efficiency.
http://en.wikipedia.org/wiki/Fork_end
other than that, any old frame from a reputable bike manufacturer is fine. and most of them make a single speed frame to cash in on the fad.
see if you can get a bike with a flip-flop hub. fixed in one direction. flip the wheel, single speed free wheel on the other.
i got a road bike first and later added a fixed to my stable. most of my commuting is on fixed because 1) it's fairly short. 2) it's waaaaaaay more fun. my road bike is overkill for stop n' go traffic without any stretches to wring it out. i've been entertaining thoughts of commuting on bmx...