AT&T iPhone Tethering to Cost $30/month?SAccording to a MacBlogz source, AT&T's upcoming 3G iPhone tethering plan will run $30 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 $0.00048/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]