Apparently the attack vector is pretty small considering:
http://www.digitaltrends.com/mobile/femtocell-verizon-hack/
Fortunately for Verizon customers, the company has since issued a patch to all affected femtocells. Sprint currently offers a femtocell that is similar to the vulnerable models from Verizon, but the company has said it plans to discontinue the device. And while AT&T also offers femtocells, it requires an extra level of authentication that makes much of the iSEC Partner’s findings irrelevant. Still, says Ritter, the femtocell vulnerability is a major problem.
And
Ritter suggests that all carriers that offer femtocells require owners to provide a list of approved devices that are allowed to connect to their femtocell. And also prevent customers’ cell phones from connecting to any unauthorized femtocell.