You know, T-Mobile, a few years back, introduced UMA (Unlicensed Mobile Access) with some of their phones (which T-Mo has subsequently marketted under 3 different names, you know, to confuse their customers, I guess), but none of the other carriers picked up on it, and T-Mo pretty quickly abandoned it - I believe their network still supports it, and some/all of their Blackberries support it, but they pretty quickly stopped advertising it, none of the Android phones support it, and T-Mo has quietly gotten rid of every non-Blackberry phone that used to have the UMA feature.
It's really kind of a shame - UMA is a great idea: basically, any WiFi hotspot that you can connect to become a "cell tower" (well, it routes cell phone traffic over a tunnel on the Internet, to T-Mo's network, so it basically becomes VoIP). This Femtocell idea is something that some of the other carriers are sort of testing (I have some relatives on Sprint who got one because there is very poor reception at their house). But, I think UMA is a superior solution to these femtocells, because a) with UMA, you need a phone with UMA support, but you had to get a phone anyway, so adding UMA to phones would have been almost 'free' from the customer perspective, with the only other equipment needed being something you *probably* already have, and if you don't, you can get dirt cheap at Microcenter, Best Buy, Fry's, etc., and B) the femtocell will *only* work at your own location where you put it, whereas UMA would work with any Internet connection and most Wifi hotspots, which means that I could take advantage of it at other locations if they have WiFi (relatives or friends houses, school, work, shopping, etc) too.
Now, I think with the Android phones, you can now do some VoIP calling, but the advantage with UMA was that calls would seamlessly transfer between wifi and the cell network (if you left Wifi range, or entered Wifi range). It's really a damn shame that the cell phone industry didn't adopt UMA as a feature, because to me, it seems like a vastly superior approach than femtocells.
I suppose it's theoretically possible that UMA could rise from the ashes, but at this point, it seems kinda dead. More's the pity.