Even if you could simply swap your SIM card to swap providers in the US, the majority of customers would not suddenly become portable; people don't want to pay $500 up front for a phone, even though they're paying more down the line.
Even if iPads or other tablets become subsidized by carriers, my guess is that a lot of tablets will still be sold without subsidy simply because people are used to it.
Another effect of being able to simply swap SIM cards is that there will be a larger population of useful old unlocked/unlockable phones out there with expired contracts. A lot of early GSM competition in Europe was driven by smaller carriers or MVNOs who could simply send you a SIM card to put in your old phone.
As far as I can tell, T-Mobile is (was?) the only major US carrier to even offer the option of buying a phone up front and then getting a month-to-month plan that costs less than the equivalent 2-year contract. That is what I use and all told it is going to save me about $200 over two years vs. a subsidy based plan. But if hypothetically my monthly rate were the same as what it would be on a 2-year contract, as it would be on AT&T, why wouldn't I take advantage of the subsidized phones (apart from lock-in)?
OK, there are prepaid plans, but the choice of phones (especially smartphones) is limited compared to what's available on postpaid.
It crashes in call (total power off); data availability hasn't been stable (I would say about once a week I have significant issues where I won't be able to use email, a browser, etc. for the entire day); maps/GPS is iffy; I think there are some interface design flaws.*
I used to have an iPhone 3G. I thought it was great.
I switched to VM for the plan -- $25/month for 300 minutes, unlimited text and data ($40/month for 1200 minutes).
Honestly, though, these problems haven't really bothered me too much. I've found that for $25 or $40 a month it's fine with me if the experience isn't "perfect".
*My friend also experiences these same problems
Search #vmdd (Virgin Mobile Data Down) on Twitter and follow the fun.
The Optimus V is a late 2010 phone. My iPhone 3G is from 2008 and hasn't been sold by Apple for a full generation. Your comparison doesn't really make sense.
Not that they're that expensive new. But if portability was easy there would even be an omnipresent resale market for used phones, so even boring phones retain resale value, as is common in other countries.