According to a support page on Apple's own website (Locating iPhone wireless carriers) several carriers over the world sell unlocked iPhones.

I've compiled a list here for your viewing pleasure but the Apple website may be more up to date:

• Belgium - Mobistar
  16GB: 575 EUR
  32GB: 675 EUR
• Czech Republic - O2, T-Mobile, Vodafone
  16GB: 15 777 CZK
  32GB: 18 377 CZK
• Greece - Vodafone, Cosmote
  16GB: 569.00 EUR
  32GB: 668.99 EUR
• Hong Kong - 3
  16GB: HK$ 5,388
  32GB: HK$ 6,288
• Italy - TIM, Vodafone
  16GB: 619 EUR
  32GB: 719 EUR
• Luxemburg - Vox Mobile, LUXGSM
  Unlocked, but there doesn't seem to be a way to buy without contract... correct me if I'm wrong.
• Macau - 3
  Unlocked, but there doesn't seem to be a way to buy without contract... correct me if I'm wrong.
• Malaysia - Maxis
  16GB: RM2490
  32GB: RM2990
• New Zealand - Vodafone
  16GB: 1,179 NZD
  32GB: 1,379 NZD
• Russia - MegaFon, MTS, Beeline
• Saudi Arabia - Mobily
• Singapore - SingTel
• Slovakia - Orange
• South Africa - Vodacom
  16GB: R7500
  32GB: R8500
• Taiwan - Chunghwa Telecom
• Thailand - TrueMove
• Turkey - TurkCell
• United Arab Emirates - Etisalat

The prices in the list above are for the 3GS, purchased without a contract (either outright, or with a prepay SIM) and factory unlocked. If you have some info for your country, please post or PM me and I will add it to this list.

*NOTE* Most of these confirmations are for the iPhone 3G, but the assumption is that not much will change with the 3GS.
UPDATE: Apparently my assumption was correct as the Apple support page has now been updated to say that it reflects the 3GS as well.
UPDATE 2: Added lots more prices and links.

I assume they are not unlocked, otherwise it seems to me that unlocking an AT&T iPhone should be as simple as getting one of the "unlocked" ones from one of those carriers and diff-ing the filesystem?

Originally Posted by risner

I assume they are not unlocked, otherwise it seems to me that unlocking an AT&T iPhone should be as simple as getting one of the "unlocked" ones from one of those carriers and diff-ing the filesystem?

They are unlocked. Unfortunately, it's not as simple as you think.

As Volkspost said: "To unlock an iPhone you need the NCK which is a unique 15 digit key. Those keys for unlocking are sitting on Apples servers and send to the iPhone via iTunes while the iPhone is connected to the Mac/PC.
Each iPhone has a unique HWID, NORID, CHIPID, (id's embedded in the iphone hardware/chips and unique to each phone), the NCK is only working with the one iPhone where the above are matching. The NCK does not unlock any other iPhone.
Trying to bruteforce the NCK would take years even on high-end computers (NCK Brute Force - The iPhone Wiki).
Trying to guess the NCK is limited aswell, After 5 or so unsuccessful attemts, the iPhone becomes permanently locked to the carrier - unless you're feeling really, really lucky, I wouldn't try it.

Apple has HWID, NORID, CHIPID... of all iPhones sold in countries, where the iPhone has to be sold unlocked (About activating iPhone 3G with a wireless carrier). So once a phone of those is connected via iTunes, the apple servers check HWID, NORID, CHIPID and compare it to their database. If the matching iPhone is marked "factory unlocked" the Apple servers send the unique NCK for this iPhone."

Originally Posted by duck9000

They are unlocked.

Has it been confirmed that the phones in all those countries are actually unlocked though? Because I'd only heard of Italy, Hong Kong and Belgium before seeing the support page.

Originally Posted by Rekoil

According to a support page on Apple's own website (Locating iPhone wireless carriers) several carriers over the world sell unlocked iPhones.

I've compiled a list here for your viewing pleasure:

• Belgium - Mobistar [CONFIRMED]
• Czech Republic - O2, T-Mobile, Vodafone
• Greece - Vodafone, Cosmote
• Hong Kong - 3 [CONFIRMED]
• Italy - TIM, Vodafone [CONFIRMED]
• Luxemburg - Vox Mobile, LUXGSM
• Macau - 3
• Malaysia - Maxis
• New Zealand - Vodafone
• Russia - MegaFon, MTS, Beeline
• Saudi Arabia - Mobily
• Singapore - SingTel
• Slovakia - Orange
• South Africa - Vodacom
• Taiwan - Chunghwa Telecom
• Thailand - TrueMove
• Turkey - TurkCell
• United Arab Emirates - Etisalat

What I would like to know is if it is true that all these carriers sell unlocked iPhone handsets?

cyprus also sell them factory unlocked ( 32gb and 16GB but for the 32 its 1099 euros and 16 is 999 euros.. bit pricey but they are factory unocked)

The ones in the UAE were unlocked (3G), we will need to find out if the same applies to the 3GS in august when its released.

Cheers

BEn

Greece has also factory unlocked iphones

This is looking more and more like that list is in fact true. Awesome! The critical one I'd like confirmation on is Russia, because it seems unlikely for some reason hehe. (or maybe this was a strategic move on Apple's part, less Russians with locked iPhones means less Russians attempting to unlock it )

Originally Posted by Rekoil

This is looking more and more like that list is in fact true. Awesome! The critical one I'd like confirmation on is Russia, because it seems unlikely for some reason hehe. (or maybe this was a strategic move on Apple's part, less Russians with locked iPhones means less Russians attempting to unlock it )

Well, of course this list is true. Apple has been selling factory unlocked phones in certain countries for quite some time, this is not news. Unfortunately, these phones are prohibitively expensive, just look on ebay for the going rate, its usually above $1000USD just for a 32GB 3GS.

Originally Posted by thecompkid

Well, of course this list is true. Apple has been selling factory unlocked phones in certain countries for quite some time, this is not news. Unfortunately, these phones are prohibitively expensive, just look on ebay for the going rate, its usually above $1000USD just for a 32GB 3GS.

Ok, thanks! I only knew of Italy, Belgium and Hong Kong, I hadn't heard of any other countries selling unlocked iPhones. I guess that makes it easier for me to get one. See in Sweden you end up paying just as much if you buy it from our local carrier Telia (after the 2 year lock-in period at least). So I'd rather buy a factory unlocked one and use it with my cheaper carrier Tele2 instead of Telia.