I work in central London and see a lot of people riding about on these bikes, clearly very expensive ones often too. This is a part of town where it's impossible to park, the traffic is terrible, and you have to pay[1] to drive there at all. I definitely see the fixie (or really the bike in general) taking over from the car as a status symbol for some people. Driving a car through Soho marks you out as either rich or stupid or both, a bike has a whole set of other connotations about fitness, ethics, intelligence. The expensive ones also carry wealth sat us of course.

I think the point is, that while it's true they are often status symbols, there is a practical motivation behind them too.

[1] Congestion Charge - http://www.tfl.gov.uk/roadusers/congestioncharging/