You don't think millions of folks, including the many thousands that hog bandwidth just for the sheer fact that they can won't drag down Verizons data network as it did AT&T's??? I can't wait for Verizon's iPhone rollout. Then, AT&T's network will free up.
In any case, Verizon sucks where I live, but AT&T has ample coverage. So for myself and many others who plan on staying on AT&T for the foreseeable future, not upgrading to iPhone 4 would be completely nonsensical. I can sell my 3GS for more than the contracted iPhone 4 price. Since I'm staying on AT&T anywhere, I will make money on upgrading so will thousands of other people.