I ride fixed in San Diego. I'm less than a year into riding fixed, so take what I say with a grain of salt.
From what I see, San Diego is mostly about roadie/triathlete racing culture (lycra/carbon) or mountain biking (full squish/camelback). As far as urban riders, most are on cheap mountain bikes, but I'm seeing an increasing number of old-school road bikes - still, those are geared.
If you talk specifically about single speeds, I'd say 99% of them are beach cruisers. You see plenty of track bikes at the Velodrome, but that's because... it's a track. Urban fixed gear riders are rare, and mostly bike nuts of one kind or another.
I think San Diego is just too damn laid back to generate much of a scene. Hell, even in our sketchiest neighborhoods, it's rare to see a bike locked up with much more than a thin cable. It's as if the thieves are too mellow to do much of anything.