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Basically if your iPhone is officially unlocked (either factory new unlocked, or unlocked by O2 both work) and you use the SIM of a network who do not have a built in carrier settings IPCC file (eg in the UK, only T-Mobile or 3, although it may work with MVNO's like Virgin as well) then the Cellular Data settings section has a new section after MMS called Internet Tethering with the fields APN, Username, and Password.
With no jailbreaking, no installation of custom IPCC files or Mobileconfig's, I was able to simply enter three.co.uk into the APN field for tethering, turn on tethering, and it worked straight away
I was also able to enter a different APN to the main Cellular Data APN setting - for example I tested 3internet as well as three.co.uk.
This is fantastic news for those with unlocked iPhone's on unofficial carriers, as Apple received a lot of heat from customers by basically yanking the ability to tether on unofficial networks when 3.1 came out, after the feature being introduced in 3.0. (A lot of people saw it as a bait and switch where an advertised feature that they might have bought the phone expecting to work was disabled by Apple with a firmware update with no warning and no way to reverse the firmware upgrade to get the feature back...)
Unfortunately at the moment it doesn't seem possible to enable tethering with a custom IPCC, so those of us on unofficial carriers who use an IPCC to customize the phone for that carrier will either have to choose between that and no tethering, or tethering but no customization. (I am still trying to figure out if existing IPCC's can be updated though)
Also this doesn't help anyone on official carriers like O2, Vodafone, or Orange - because those carriers have built in IPCC files in the OS they take priority, so you will not get the manual fields in Cellular Data for those carriers - thus this can't be used as a workaround for enabling tethering without the carriers consent.