Apple initially went to Verizon with the iPhone, but Verizon wouldn't agree to many of the conditions Steve Jobs wanted. So Jobs went to the next largest network, that being Cingular (now AT&T). AT&T accepted the conditions Apple set forth, or probably some variation (Apple probably had to consent to a few changes because there was no way T-Mobile or Sprint were viable options, and Jobs couldn't go back to Verizon after having snubbed the largest carrier). Some of the concessions I can think of are the five year exclusive deal and the crippled Bluetooth, and who knows what else.
So don't expect an iPhone to come to another network for another four and a half years. Plus, even if Verizon went ahead with its open network access plans, the current and potential 3G iPhone still wouldn't work with it as you would need a completely different cellular radio to make it physically possible.