VoIP Peering is going to be a hot subject really soon, and not only with the VoIP loyalists.
I expect this to be a major business sector with the roll out of municipal wireless hotspot clouds in cities around the USA and the rest of the world.
If you think about it, as Google, Earthlink and others build out the wireless cities, the opportunity to roam with a WiFi phone will emerge. This is going to be like cellular all over again.