We've had this for a while (Score 1)

Airespace (www.airespace.com), recently acquired by Cisco, has had this functionality in it's wireless products for some time now. They claim roaming between their access points does not adversely interfere with VoIP call clarity. Obviously, this is designed to work within a single network that has twenty or thirty APs connected to the same network that cover an entire building, not your neighborhood which has a few APs connected to diverse ISPs which are scattered around the block. ;-)