T-Mobile's HotSpot @Home already brings WiFi VoIP to compatible cell phones, and a recent FCC filing from T-Mobile and Linksys indicates that soon all the phones in your home will be able to get in on the action. The WAP, from Linksys, is the WRTU54G.
From CNET (http://reviews.cnet.com/cell-phone-and-smart/t-mobile-hotspot-home/4505-6448_7-32486222.html?tag=txt):
"When you first sign up, you'll receive a complimentary wireless router that's optimized to be used with the Hotspot @Home service. Our review model was a Linksys, which is exactly the same as the Linksys WRT54G, except for a few software tweaks designed by T-Mobile."
I went looking for the source for T-Mobile's modifications but came up with nothing. The Linksys device, as you know, uses GPL code. The T-Mobile modified version of the Linksys device does as well and therefore one wonders if T-Mobile will eventually release the code or remain in violation of the GPL.