Having spent ages researching this myself last time i was in the US and was looking to buy one, heres what i found out.
There are 2 models in the US
1. Model no A1428 - unlocked and bought direct from apple stores. it is unlocked and will take any 3g nano sim, so you can bring it home and use it in the UK as well on any 3g carrier. BUT
it will only do 4g on the following frequencies 4 (AWS)
7 (700b MHz)
These frequencies are only found in the USA, Canada. Mexico and Puerto Rico.
2. Model no A1429 - This is the CDMA version for Verizon. It can be bought from Apple stores but they normally only sell it with a contract, some stores will sell it off contract and some won't. Some Verizon stores will also sell it off contract but again it's store specific.
But it also has a sim slot which accepts a nano sim. It will work 3g anywhere as the A1428 but only 4g on the following frequencies 1 (2100 MHz)
3 (1800 MHz)
5 (850 MHz)
13 (700c MHz)
25 (1900 MHz)These frequencies are used by USA and mainly Europe, far east, and down under.
The interesting frequency is number 3 (1800mhz) as that is what EE currently use.
This phone would give you 4g in the UK with EE only
. EE also plan to use other frequencies in the future which are not on this phone. Also as yet the other UK providers haven't announced their frequencies, so it may be compatible later on and it may not.
So a gamble really, buy the A1429 and use it 3g anywhere and only on EE here, or the A1428 with 3g anywhere and no 4g in the UK.
Full details can be found here http://www.apple.com/iphone/LTE/