Right now I'm working with an old 10-speed style hub converted to SS, but I just ordered a wheel through the local shop that is a cassette-style so I have the option to go fixed with that surly unit or SS with a SS sprocket and some spacers. We'll see how that works out, I've been on plenty of single speeds, but no fixies so I might just do it to try it. I have semi-horizontal dropouts though (1-2mm of play) so it might work better as a SS unless I can work a gear ratio on it that I like a lot.