Apparently O2 has already offered this in 2009. Anyhow, it's a good news for every iPhone users especially for those who are planning to upgrade to iPhone 4 with the intention of using their current iPhone with other carriers.
I think people will appreciate it if I explain briefly what sim-locks are. According to wikipedia, sim locks are capability (codes) built into gsm mobile phones that prevent their usages from other carriers. Basically.
Generally, there are 3 types:
1. Mobile Country Code: the phones can only be used in 1 country.
2. Mobile Network Code: the phones can only be used with 1 carrier. i believe this is the
commonest one. some of them are very restricted, viz., absolutely no
access to the phone from other carriers. some are less restricted, in
Malaysia, some phones come with labels on it like vodafone, 3, etc.,
the users can use this phone with Malaysia carriers but with possible
restriction on internet access eg gprs, edge and 3g.
3. Mobile Station Identification Number: the strictest one, only one sim is allowed.
I believe sim locks are softwares in nature, in other words, you can unlock it with counter-softwares. The most popular unlocking software for iPhones is Jailbreak. Unlocking iPhones with Jailbreak is a rather straightforward process where you can learn from youtube or send your iPhones to the shop for a minimum waiting time.
Anyway, for those who wish to unlock their iPhones, it can be done over the air, viz., you don't need to send your iPhones to O2. Just fill in the unlocking form and wait for within 2 weeks time (I unlocked my iphone in just 2 days time).
It should be noted, iPhone 4 is the exception to this. For iPhone 2g, 3g and 3gs, you can unlock them ANYTIME but of course, you will still need to pay for the remainder of your contract.
Unlocking for pay monthly customers is free but costs GBP15 for pay and go customers.
Lastly, here is the link which include another link to unlocking form: