I wrote about them last April
They are becoming more mainstream as this MIT Technology Review writes
"In effect, every customer who buys and installs his own home femtocell would reduce the load on the carrier's local macro network. The femtocell itself serves as an alternative base station, broadcasting and receiving ordinary wireless signals from cell phones that the femtocell owner permits. This is a strikingly attractive idea, particularly to carriers in big cities that find their networks often overloaded, and find that local regulations or public opinion makes it difficult and costly to set up new antennas."