...Now they are called "fixies".
Several months ago, I went to a concert and saw a bunch of fixed-gear bicycles chained up outside the venue. Since I had been out of the USA for about two years, I just chalked it up to another change in culture that had happened.
Then I came across this link to the phenomena via kottke.org.
My theory is that it's just a just a fad thing. People love to have the latest thing. (I fall into that category sometimes.)
Way way back - when I was in high school - I rode one. It was made by Raleigh. It weighed about 18 lbs. That's really light. I could feel every single stroke of my legs being transmitted to the pavement. So I can see the attraction.
A word of advice - if you didn't read the linked article and you're going to buy one, get a handbrake installed. Also, you can get a freewheeling single gear, so that it's not fixed.
I'll just stick with my old mountain bike. It works fine. (I have a road bike, but we had a mouse invasion of the shed it lives in a few years back - and I haven't gotten around to cleaning that bike up! The mice left their mark, shall we say.)