Apple has posted an FAQ (frequently asked questions) page concerning the new SIM-free (i.e. unlocked) iPhone 3GS and iPhone 4 they're selling for the first time in international Apple Store like UK, France, and Canada.
Bottom line? They're really, truly, 100% unlocked and will work on any GSM carrier for which you have a miniSIM (iPhone 3GS) or microSIM (iPhone 4). You'll pay full price for them, of course, but for those who absolutely don't want a contract, or for frequent international travelers, this is huge.
(Note: No T-Mobile US 3G. Sorry folks, even though iPhone 4 is pentaband, T-Mobile's 3G band isn't supported. It hurts. Sorry).
Here's the cover:
Buy your iPhone from Apple commitment-free.
It works with the carrier of your choice. Buy your iPhone from the Apple Online Store and choose your own GSM carrier. You can change carriers at any time.
It's easy to set up. Get a compatible SIM (iPhone 3GS) or micro-SIM card (iPhone 4) and select a plan from the carrier of your choice. Then insert the card and set up your iPhone using iTunes on your Mac or PC. With a few clicks, you can start making calls.
It works internationally. Because this iPhone is commitment-free, you can purchase a micro-SIM or SIM card and service from a local carrier at your destination. Or check with your home carrier regarding international roaming charges.
Full FAQ after the break!
[Apple.com iPhone 4 FAQ, via @havvac]