It seems the concept of trusted partners is becoming more en vogue. While in theory any SIP based VoIP service can run over any IP network I'm sensing a move in the direction of trusted partners to avoid problems and more importantly really add teeth to the idea of "we don't support that" by adding "but we do support X that works really well with our network."
While with software like e-mail and browsers that are standards based that song doesn't sing real well with me, in an era where so many new players are in the emerging voice services space, a startup like Clearwire can only support so many. Hence this idea of Trusted Partner makes sense.
It's not much different than what client PhoneGnome has announced with VocalNet as an officially supported ITSP for long distance in their Equal Access program.