Okay-- I've been riding singlespeed/fixed road bikes for about six months now. The first was an old Bianchi road frame from the trash with a bad green paintjob. I re-used everything I could-- stem, BB, cranks/chainwheel, brake lever, etc. The only new parts were the cheapest Suzue/Alex wheelset I could find on eBay-- $135 I think. The whole project, including tires, cost about $200.
Prior to that I rode my really-nice Voodoo MTB around town. Front shocks, nice Mavic rims, full Shimano XT package. If I'd have bought it new it would have run at least $1500.
Guess what bike attracts the attention of sketchy mother****ers? My fixie. Nobody blinked at my Voodoo-- it was one mountain bike in a sea of similar-to-the-untrained-eye mountain bikes. The creepy guys hanging out in the shadows, on the other hand, seem drawn to any bike with skinny tires and one gear.
"Nice bike," they say. "That's American made, right?" (um, try Taiwain.)
"Hey, do you race or what? That's a slick bike."
I rarely feel like my expensive mountain bike is vulnerable-- but the fixie is a target. The wheels are cheap and the frame is from the trash, but the common theif probably just sees "race bike."
The rear wheel/crankset theft story below makes a whole lot of sense.
(or perhaps all these urchin types are retired velodrome racers. that must be it.)