Attention US iPhone users — it looks like AT&T and Apple are at last making good on their long-held promise to offer tethering support through the iPhone, but not until the release of iPhone OS 4.0 later this summer. This illustration comes from inside the iPhone OS 4.0 beta that’s circulating at present, though the number given takes you to a recorded message saying “AT&T doesn’t offer tethering at this time”. Tethering — the process of using your iPhone as a 3G modem with your Mac — is really useful when traveling, or as a home back-up Internet connection for when your broadband goes…