1. By locking in with a&tt they got concessions that Verizon was not willing to grant, which must have had some effect on the phone's popularity. (Visual voice mail, no carrier mark on the phone, a very good subsidy (though, initially, no subsidy)...)

2. By only targeting GSM, they drastically simplified the engineering required for the device. The recent comments from an anonymous engineer on the Pink project made it clear that targeting both GSM and CDMA was a huge mistake for them. My understanding is that the two radios are not simply interchangeable. Building an iPhone for both would have required coming up with two radically different internal designs. This would have required engineering hours which would likely have delayed the phone. (or the 3g, or the 3gs) And that delay would naturally have cost market share.

So, it is pretty hard to say what would be different if Apple had a phone that ran on other carriers. And, it is a two way street, it sure doesn't look like Verizon plans on having the iPhone any time soon.