I'm pretty much with Eden on this one - I figure there really isn't much I could do to stop somebody stealing my bike if they really wanted to. I use a cable lock on my commuter/errands bike (when leaving it outside shops and stuff - I am allowed to keep it in my office at work). But I do so in the knowledge that it will only stop the really opportunistic thief who would see an unlocked bike and run off with it.

I try to avoid leaving my good bikes (my mountain bike or my road bike) unattended. When BF and I ride together one of us will mind the bikes if we need to go into a shop or whatever - and we will try and eat at outdoor cafes where we can have the bikes next to us. Sometimes when I am riding alone I leave my bike unlocked if I need to stop at a restroom - but I try to avoid it (and I make it quick )