AT&T's has just pulled the shroud off their first femtocell device (a box that produces cellphone signals at home). It's called the 3G MicroCell. And we have a few details regarding how it will work.
The 3G MicroCell uses your home's/office's internet connection to support up to ten 3G cellphones, though only four of those phones can stream data or voice simultaneously. If a fifth person wants to make a call or browse the web, they'll be redirected to their normally subscribed 3G service automatically. Calls can also automatically transfer from the MicroCell to a tower with no interruption.