Apple has started to sell Unlocked iPhone 4 officially from its web-store starting today. The price is $649 for a 32 GB model and $749 for a 64 GB Model. Unlocked iPhone basically gives you the freedom to choose your own service provider without any contract and you can change your service provider anytime you want without worrying about any penalties being imposed by service provider.
Obviously a unlocked iPhone is costlier than a locked one, but in my opinion its a better choice. The cost is high as the service provider is not subsidizing it and putting you into a long unilateral 2 years contract agreement.
As Apple states in FAQ of unlocked iPhone 4,
Buying an unlocked iPhone 4 allows you to choose your own GSM carrier, change carriers at any time, and even use multiple carriers if you travel frequently. Check with your carrier regarding the length of commitments or contracts.
Everything else remains same with iPhone compared to a locked phone.
If you want to buy a unlocked iPhone 4, you can just head over to Apple.com and here you can select either 16 GB or 32 GB iPhone 4 plus you can choose either a black or white colour.
PS:- As far as I know, this Unlocked iPhone is no same as jailbroken iPhone. It only gives you the freedom to choose your own service provider but other restrictions remain in place like you cannot use apps which are not officially approved by Apple or in short, anything outside Appple app store. Correct me if I am wrong (It may be the case as I am not a expert with Apple Products)