I've left my camp unattended fairly often. I've never had a problem, but I've met people who've been victims of petty theft. It all depends on the situation.

Put the rainfly on your tent so a potential thief has to worry about the possibility that someone is inside of it. Most people like to justify theft - "I only steal from walmart", "that guy probably wasn't using it anyway", ect. If you leave any gear, keep it stacked up and tidy so some kid can't tell himself he 'just found it'. When I leave my bike, I keep it loaded up. Stealing a bike is one thing, stealing a bike that someone is obviously living off of is a whole 'nother level of heartlessness.

All in all, I'd not worry about it too much. Just be sensible.