> "Hey, do you mind watching my stuff for a minute?" works pretty well.

It works pretty well with people who are street-smart enough and really care about other's possessions. I once trusted my purse to a friend of a friend because I had to go on stage to act. When I returned, my purse was hanging from her chair, unwatched. So, it's not about honesty, it's about people really caring about your gear and being able to look after it. Environment matters, too. That time it was a crowded place. A library it would be different.