AT&T is following in Sprint's footsteps by experimenting with in-home cell phone boosters, otherwise known as femtocells. Sprint has been selling its own under the Airave brand. Femtocells are made to boost the cell phone signal via your high-speed Internet connection; think of it as your very own cell tower.
AT&T is planning to make its own version available in a trial market next year, and hopes to improve 3G performance in addition to enhancing voice signals.
As for other carriers, Verizon has mentioned that it's looking into femtocells, while T-Mobile has gone its own route with T-Mobile @ Home, which lets you make calls over Wi-Fi. The T-Mobile @ Home service can only be used with UMA phones that can switch between GSM and Wi-Fi networks, however.