Don't let femto fall to the marketing scum!
For the record - one femtometer is roughly half the width of a proton. Femto is a prefix meaning 10^-15. Therefore, a femtocell would have to cover one 10^-15th the width of a regular cell. GSM can reach (optimistically) 50 km from the base station, so we have (5m*10^4) * (10^-15) = 5m*10^-11 = The width of the inner electron cloud of a carbon atom. I can't imagine a cell zone of this region being useful, but I certainly give props to the engineers who can design something this small! Please call them, say, millicells or something more accurate - and stop peeving my pet.