According to a MacBlogz source, AT&T's upcoming 3G iPhone tethering plan will run $US30 a month over existing subscriptions and have a 5GB cap. That sounds about right to us, as the plan is identical to that which AT&T already offers to Blackberry users.
They also explain that the connection will simply be cut if you go over 5GB, preventing potentially huge overage charges (BlackBerry users can opt to pay $US0.00048/KB for extra bandwidth, which would make sense on the iPhone, too). And at least the first time you begin tethering, the transaction will take place through iTunes.
As for a potential launch date, there's no word at this time. [MacBlogz]