Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
But, if the phone is "open" in this context, more of a given device can be sold and used across carriers. Thus, a high-demand unit can generate more volume and drop in price anyway.
We would probably still have the current system, but call them "bundles" with a discounted phone and plan, rather than it being a necessary evil to get better phones and/or service options.