It depends on what you mean by "landline". A traditional mobile phone talks to a base station, usually a few miles away. From there it communicates to the PSTN (Public Switched Telephone Network) just like a normal phone does. Sometimes this is a traditional connection like a T1 or ISDN BRI (most likely muxed into a SONET fiber ring), more frequently this is becoming an Ethernet connection offering Voice over IP services to the PSTN core.
With a femtocell you set up a tiny little base station right in your house. The coverage isn't that great (compared to a real base station) but it can generally cover your house a a reasonable amount of property around it. Your mobile phone talks to the base station and sets up a Voice over IP call to the wireless carrier over the Internet which then gets piped into the PSTN.
Technically you're using a physical line in your house (unless you have a wireless broadband connection) to communicate with your mobile phone, but since you can use the same phone and number wherever you are it's a pretty seamless and automatic transition from talking to the femtocell to talking to the public base station. You should be able to make a call in your kitchen and then drive to town without having it drop the call, but that depends on how cleanly they've implemented things (the CDMA technology behind Verizon and Sprint's network should handle this better than a GSM-based AT&T network).