Here's my latest fixed project: a fixed Dahon Boardwalk S1. My full sized bike is here: http://www.fixedgeargallery.com/2006/jan/SeaBiscuit.htm
My commute occasionally involves busses across a bridge without a bike lane and I've been stuck for more than an hour waiting for a bus with an empty slot on the rack. Naturally I developed an interest in folding bikes. A brompton's a bit out of my wallet's reach, so when a singlespeed dahon frame + rims came up for a benjamin I jumped on it.
The wheels were my first hand built set, and they've worked out nicely thanks to cheap, detailed instruction from Roger Musson. The rims are velocity deep-v aeroheat, laced to some generic track hubs with 18 straight guage spokes (they don't make double butted's so small) in a crow's foot pattern. Schwalbe 'Marathon' slicks on the rims.
Cranks, sprocket, and chain ring are all shimano ultegra, as good as I could get on a small budget. Front brake is also Shimano, XT which was left over from another project. I added some pedals, moustache handlebars, and a cyclocross style brake lever, and a selle italia saddle. The plastic 'patch' on the rear seat-stay is to protect the frame from rubbing up against itself when folded.
Handling's not great, the handle-post is quite flexible, and she's a bit squirrley on the road. All in all she gets the job done -- drivers let me on the bus with the bike folded up no problem. Coming up: a better saddle, fenders, and a new handlepost.