It's a little odd to be walking down the street with the knowledge that there's only one person behind you and to hear their voice talking to somebody.
If you turn around and there's just the one person then it's understandable if the person has mental problems, or they've obviously pulled out a cell phone and they're talking on it.
I was just down at the DOL office with about a thousand people, and a few of the folks there had those blue tooth things strapped to their ear. Some times they'd be talking to somebody in the office, but other times it wasn't obvious who the hell they were talking to. Other people had cell phones and those were a lot less freaky.
If you're going to talk on the phone make sure other people in physical proximity can tell with one glance what you're doing.