Comcast is transforming its customers' home modems into public Wi-Fi hotspots by adding a second signal to each device. In addition to a customer's home Wi-Fi connection, Xfinity wireless gateways (which include the cable modem and wireless router) will by default broadcast a separate signal that other Comcast subscribers can log in to with a Comcast username and password.

Comcast described the program today, saying that it has already been piloted in Pennsylvania, New Jersey, Northern Virginia, and the Greater Washington, DC metro area.

"Comcast’s newest Wireless Gateway broadcasts two Wi-Fi signals," the company said. "By default, one is securely configured for the private use of the home subscriber. The second is a neighborhood 'xfinitywifi' network signal that can be shared. This creates an extension of the Xfinity Wi-Fi network and will allow visiting Xfinity Internet subscribers to sign in and connect using their own usernames and passwords."

Since the networks are separate, customers don't have to worry about losing bandwidth to passersby, Comcast said.

"Our broadband customers will continue to get the service that they are paying for," Comcast Senior VP of Business Development Tom Nagel told CNET. "That was extremely important to us in designing this product."

The program is similar to one used by BT. "[A]ll BT Wi-Fi members securely share a portion of their wireless bandwidth through a separate channel on their BT Home Hub or wireless router with other members who are in range of their Hub or router," BT says on its website.

There doesn't seem to be much advantage to individual Comcast subscribers when they're at home, since anyone with a home wireless network can already set up guest access for visitors. But subscribers could potentially benefit when they're out of the house and happen to be near another Comcast subscriber's wireless gateway. Through a partnership with the CableWiFi Alliance, Comcast already has a network of more than 150,000 hotspots in more than a dozen US cities.

The idea of turning home Internet connections into Wi-Fi hotspots has also been advanced for the purposes of emergency communications. One proposal we wrote about last year would use wireless guest networks to give first responders access to all the routers in the area to enhance the capabilities of mesh networks. After the Boston Marathon bombing, former FCC Chairman Julius Genachowski and Harvard Professor Jonathan Zittrain proposed using Wi-Fi access points in homes and businesses to provide public access during emergencies.