Apple has started selling the unlocked GSM iPhone 4 that you can use with any GSM network. The 3G bands haven't changed, folks! So you won't be getting 3G download speeds on T-Mobile US for example, but it's nice to not have to jailbreak and unlock an iPhone 4 just to use it on T-Mobile.
The unlocked iPhone 4 from Apple sells for $649 for the 16GB and $749 for the 32GB and requires no contract. For more info visit: http://store.apple.com/