Those of us with bad cell coverage at home can't wait for this privilege of paying twice for a fix (lending our wired broadband backhaul to the wireless company from whom we're buying service). Meantime, there are some other interesting opportunities that ubiquitous femtocells could provide.
Since the femtocell can identify your phone, and you usually have your phone with you, the femtocell owner can tell when you're nearby. In a big facility, like an airport or conference center, the owner has a pretty good idea of where you are.
When I worked on RFID-based airline check-in at eTag Technology, we built our initial system using passive RFID chips embedded in loyalty cards. But we always planned on leveraging femtocells or similar technology to detect passenger arrival (and location) in the terminal. Just give the airline your phone number, and you can have your check-in, upgrades -- even rebooking if you're not gonna make it -- handled automatically. Eliminating the custom hardware and RFID tag issues makes the concept even more of a no-brainer.
At Voice Media Lab, another startup, we were leveraging the cell phone for voice recognition, media browsing, and control functions. But we wanted to use femtocells to handle the issue of visiting a friend (or a hotel, or a conference room) and wanting to play your movies on the local home theater / media system.
One idea is to wait for even big high-def media collections to become portable enough that you can put everything on an iPod. But our approach was to use your cell phone as your presence token, so that anywhere you go with your phone, all your stuff is accessible. When you go home, everything you haven't explicitly left behind (shared) disappears with you.
Of course there is a downside to all this: if your phone "just works" (without being paired Bluetooth-style to a base station network) then you're giving away a lot of location data to third parties all the time. This concern is not a trivial one, especially since a central company could collect this data, paying femtocell providers a small amount for every "sighting" of your ESN/IMEI.
(Interestingly, the same issue applies to WiFi cards and base stations, but has not been a big concern, presumably because the WiFi device is less "intimate" than a phone and because MAC addresses can be spoofed more easily than ESN/IMEI data.)