I know that iPhones purchased here come factory unlocked. Apparently in some other countries (e.g. UK, Brazil), service providers will, for a fee or free, unlock iPhones purchased elsewhere. This is done by adding the phone's unique ID to the Apple database of "officially" unlocked phones, which presumably only a service provider is going to be able to do. See http://www.iphonedownloadblog.com/2010/05/06/official-iphone-unlock/ and responses for details.

So, has anyone here approached True/AIS/Dtac to find out whether they have the power to do this, and whether, for a fee or otherwise, they will do so?

I have a 3G I brought with me from the U.S. running iOS 4.0, jailbroken, unlocked, and working fine with an AIS SIM, but I would like to upgrade to 4.1, and also not to have to worry about future upgrades. I would certainly think it worth a modest fee to convert it to "officially" unlocked status.

They can't, and they won't.