While the possibilities presented by Whrrl are exciting to many, its mass appeal has yet to be established. First, the location data might not be fine-grained enough to be useful in all cases, so it could lead to false positives. The iPhone relies on data from Skyhook Wireless, a company that uses an enormous database of the locations of Wi-Fi base stations to locate a person within about 30 meters; GPS, however, could do much better. Also, Whrrl is most useful when members of the user's social network actively contribute reviews. This requires that the user's friends have smart phones--and the motivation to critique the places they go.

Still, the biggest obstacle faced by services like Whrrl is privacy concerns. Vengroff points out that users control whom the program lists as their friends, who can read their reviews, and who can see their physical locations. The software also offers a "cloaking" feature that lets a person become completely invisible to his or her entire Whrrl network.

"Generally, if you give people more control, they're more willing to participate," says Tanzeem Choudhury, a professor of computer science at Dartmouth College. However, some people are still concerned about how long the company will store information about its customers' locations. Choudhury says that these first-generation services will likely be used by small groups of early adopters who are more aware than most of potential privacy risks and will push companies to confront them.

Regardless, Choudhury and others are excited about the potential of services such as Whrrl. In the future, she suspects, location-based services will include more predictive features. For instance, instead of explicitly requiring you to write a review, the software might recognize how often you visit a restaurant and infer that it is a favorite. "Eventually, I think that a whole lot of exciting technology will emerge that figures out how to reduce the burden on the user," Choudhury says. "There will always be the case where user input will be important, but when we find the sweet spot, that's when I think it will take off."