The NY Times has a city bike blog called "Spokes," and it does great job nailing down the issues and concerns that bicyclists face day to day in the city that never sleeps. I just finished the latest article on the blog, a piece on bike parking, and found it to be a nice articulation of a daily commuting cyclists' parking needs. The moral of the story: always lock up your bike.
Still I must confess...sometimes I don't lock up my bike.
If it has gears and freewheels, you can be sure I'll handcuff that bad boy to the nearest set of iron bars (for a lack of bike racks in our fair city). But if it's single speed and its fixed, I'm likely to leave it un-locked if I can keep an eye on it.
Because I've always dreamed about watching a bike thief steal my fixed gear. They're unlikely to recognize the bike as a fixie (at least in Memphis), and as a result they're likely to be surprised by the catapult like motion in their legs that accompanies the backside of the first pedal stroke.
Calmly walking to the bike and the forlorn thief splayed on the ground (in my dream), I would shake my head and wag my finger. "Now don't you feel silly and ashamed" I would say as I collect my bike.
In the real world, who knows if it would go down like that.
A better policy, I think, is to lock up your bike; every time.