Dopplr is a service for frequent business travelers that helps them let their friends know where they are. It’s popular. Wikipedia’s founder says it’s one of his favorite sites. Here we get a demo and hear from the geek behind the site, CTO Matt Biddulph, who gives us more details about how the service works and what you can do with it. I interviewed him from the seats the the LeWeb3 conference where they turned on the service for the first time.