You can buy the iPhone 4 for full price and then choose what carrier you want.
$200 comes from the fact that the carrier subsidizes the phone and you repay the full price ( often, much more ) throughout the life time of your contract.
It is possible to buy the iPhone outright in the USA at a full, unsubsidized price.
But generally, you have to already be an existing AT&T customer to be eligible for that option. The consequence of that decision, is that you don't have to extend your contract to buy the phone - and hence, if you were already off-contract before you made the unsubsidized purchase, you'll still be off-contract after making the purchase.
The last time I saw it worked out, if you were eligible for the fully subsidized price, it turned out to be cheaper to just buy the subsidized phone, accept the 2-year contract, and then swallow the ETF if you wanted to cancel early. That may have changed with the most recent increase in the ETF -- I haven't seen the numbers worked out since then.
However, even if you did buy the phone outright, in the USA, all iPhones are SIM-locked to AT&T regardless of whether or not they're subsidized, so you still couldn't "choose what carrier you want". Even if you could, there is a very limited selection of technologically-compatible carriers in the USA, so you wouldn't have much of a choice. In terms of carriers with nation-wide service, T-Mobile is the only company that's partially compatible, and even then you will get EDGE coverage only. (There may also be some smaller regional carriers that are also technologically compatible -- but I don't have a list.)