This came up yesterday an e-mail exchange with a friend. I noted that had I seen what the woman who called the cops on Gates claims to have seen, I would not have called. It's important to highlight the fact that Gates, and the woman, don't know each other. That said, it's understandable that she called the police. It's what people who have confidence in the police do.
I don't know how much confidence I have in the cops. But generally, unless I am sure I've witnessed a crime, I'm not calling the cops. I called them last year because some fool chained a rottweiler to the fence of a local playground, and then left. I would call them if I saw a mugging or a shooting. I probably would not call them if I saw a drug deal. And I probably would not answer any questions about any drug deal I may have thought I saw.
I think the source of a lot my reasoning is the cop's own response to Gates. A lot of us here believe that is possible that Gates was, at least, rude. We also aren't sure what--if any--role race played in all this. That said, the cop not only thinks Gates was rude to him but he handled the situation exactly right. Given that dude thinks police should be arresting citizens for rudeness, he is not the guy I'd want dealing with the kids in my neighborhood--even the ones who need to be in custody.
My basic perspective is that cops are men (or women) with guns and the legal right to shoot you, without the usual repercussions. I tend to use a lot of discretion when it comes to introducing that kind of element into a situation. It's just no way to tell how things will go down.
I don't say this as a statement of policy or advice. I think it's perfectly sensible to call the cops because you suspect a burglary. My history is my own. It's taught me something different.
UPDATE: I forgot to include this in my post, but this is the sort of thing that influences my thinking. Guy calls the police and claims a cop got shot. Police descend on the neighborhood, and catch a guy who they think is the suspect. They beat the hell out of him right there on the spot. But it turns out the guy catching the beat-down is a cop. Furthermore, it turns out now cop was shot at all. The caller lied. There'd been a murder but he/she didn't believe the police would show unless it was a cop murder. There were no charges filed. The officers are still on the force. I want as little to do with that as possible.