thbirks is right, horizontal dropouts are the key. Some older frames have semi-horizontal drops and can be used. Also, this link - http://www.peak.org/~fixin/ - has a calculator to figure out which gears you can use w/ your frame without a tensioner. They also make an eccentric axle that gives you a little bit of play if you can't make the gear you want work.

I rode a conversion frame for a while. Works pretty good, but eventually you want to dump the tensioner and strip things down to their most simple state. I've currently got a Surly 1x1 and love the geometry and feel of the ride. They run about $360 for the frame and fork. If you like steel, it's the way to go. If you like aluminum, the Bianchi looks like the best thing going. Got a chance to check out Redline's 1 speed this weekend. Not to bad for $400, but a lot of the components are pretty generic and you would probably want to replace most of them.