If you’re looking to get MMS/Tethering up and working right this second on an iPhone running the developer’s release of OS 3.0, you’re going to need to manually update the carrier file. In past builds of iTunes, performing this update was a matter of holding the shift key and pressing the “Update” button on the iPhone screen. In iTunes 8.2, however, Apple decided to do away with the manual update method – or so it seemed.
Turns out, they just obscured it a little bit. A very little bit. Seriously – it’s one command in the terminal. Kill iTunes, open the terminal (or the Window’s command window), blast in one line, and bam: manual carrier updates in the iTunes 8.2 final release.
Check out iClarified for the full details
[Thanks Justin!]