I'm finishing up my first fixie build and have ran into an issue with the chainring. I picked up a TA Specialities 48T x 1/8" chainring off ebay for around 35 bucks. the guy says it has never been used and I believe him, it's shiny as helll and doesn't show any sign of wear. The problem is there is about .5mm of play, front to back, between the chain and the chainring. Since cranks are basically levers this slop amounts to around 2-3mm at the pedals. Closer inspection of the chainring shows that the teeth have a ridge and two separate radii at the bottom of each tooth. It's seems to me that I bought a reject chainring that had been machined wrong. Ben's Cycle has some pretty decent pics of this same chainring on Ebay which clearly show a constant radius running between the teeth.
My chainring has 48 S-130 Track H3 printed on it if that's of any help.
Here's a pic of my chainring