alright well lets get our heads on straight here and try to be helpful... the OP wants a single speed, NOT a fixed gear, therefore will probably be doing some sort of road riding/training/commuting/whatever, i.e. not on a track.

so, chrisp, you can find frames that fit all of the following
and they will be, like shown in the 2nd reply, an expensive, highly specialized track frame. these frames have a very different geometry than a road frame and i would go so far as to say are totally impractical for anything other than the track. now should you get, as you mentioned, a road frame (i believe the spacers you are referring to are for the rear cassette, no?), you can simply run a chain tensioner, as mentioned in the 1st post, and you won't have to worry about magic gears, eccentric hubs, or anything else, and you have a lot more frames to choose from.

so you are going to have to make a decision between having a bike that has crazy geometry but "track ends" and having a bike that has road geometry and a chain tensioner... that's really going to be the only practical difference.

i vote you go for the later option. i have my own *budget* light-weight single speed Giant which weighs about 14 lbs and i love it. i have never had an issue with my chain tension with a properly set up chain tensioner.

not the best picture, but you get the idea.

of course, Trek is starting to make almost exactly what you are looking for, if you just want to buy complete...
http://www.trekbikes.com/us/en/bikes...istrictcarbon/