most large networks (as was said a few times I think) don't really need it in their cores. I think I've seen a nice presentation regarding the queuing delay induced on 'large pipe' networks, basically showing that qos is pointless if your links are +ds3 and not 100% full. Someone might have a pointer handy for that?
Here in London, we have noticed that the double-length bendy buses have a harder time moving through city streets than motor scooters do. I suspect that the studies you are referring to show that the key factor is the ratio between the size of pipe and the size of the flows moving through that pipe.
diffserv is the devil... and I think the voip product(s) in question aren't meant to be used in places where bandwidth is the constraint :)
A single VoIP call is a rather slim volume of packets compared to many other uses of the Internet. If a network doesn't have systemic jitter caused by layer 2 congestion, then one would expect VoIP to work fine on a modern network. Indeed, that is what Bill Woodcock reported a year or so ago in regard to INOC-DBA. --Michael Dillon