Apple on Tuesday began selling an unlocked version of its iPhone 4 from its online store.

[ad#Google Adsense 300×250 in story]Basically an unlocked iPhone can be used on any compatible network without purchasing a long term contract. This gives the user a great amount of flexibility, but the upfront cost to purchase the phone is expensive.

An unlocked 16GB iPhone costs $649; the 32GB model costs 749. There is no cost difference between the black and white models.

Apple’s Web site describes the phone like this:

“If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone 4 is the best choice. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide.”