I've made 3G calls with SIP on my Nokia N and E series phones using both Truphone and Gizmo, but on my iPhone I'm hobbled just like everyone else who uses on (that's not Jailbroken). Sure Fring works with SIP but its also WIFi.
Right now it's time for Steve Jobs to explore more deeply the concept of Openness vs. contractual closeness. SIP over 3G rocks, and that would drive data package sales for the carriers.
I'm not the only person in this camp. It seems Ted Wallingford and Ken Camp are too.