I'v said this before, my experience with at&t has been quite good, but I digress. Some observations: The Sling announcement is bigger for the iPad than the iPhone, though important to both. EyeTv 3G and others access will follow as will others. And with the happiness surrounding the iPad price points, at&t will assume that iPhone users will be happy paying a similar or slightly reduced fee for tethering. I believe it is the pricing strategy more than technical issues that are holding it up. They want it welcomed not slammed when announced.
I think we may see tethering offered for about $20 extra a month for iPhone users as well as mixed plans for the iPhone.
$15 for 250 MB month
$30 unlimited
$50 with tethering
This will likely be announced either with the iPad launch or with the launch of the next iPhone. If they want to be aggressive, they may reduce fees 1 or 2.