For all the phones that AT&T DOESN'T have an exclusive on, they allow Slingplayer. For the one phone they DO they block it on 3G. This has nothing to do with bandwith - this is about exploiting the exclusivity of the iPhone.
I think I am walking when my contract is up. If other carriers have the iPhone at that point I am DEFINITELY walking (I was a very satisfied Verizon customer for 10 years until I reluctantly switched because I wanted an iPhone). And I don't even care all that much about Slingbox. But if AT&T is going to single out people with the iPhone to treat like 2nd class citizens (no tethering plan, sling, etc), I am sure they won't stop here. We have the most expensive base price they have for a phone/data/texting plan, but we get less. Maybe they have a RIGHT to do this, but it is terrible business. They funnel all this money to Apple to save themselves with the iPhone (look at all their new customers - do they think ANY of them are their because they wanted to switch to AT&T?), and then they turn around and alienate them by refusing them services all of their other customers (and all of the smartphone customers on other carriers) get. So they're gonna have a lot of irritated customers counting the days until exclusivity ends.
This is the sort of thing that happens when there is a monopoly - iPhone lovers are a captive audience, so we have to accept a certain amount of getting dumped on. But we can also remember how we were treated, and split as soon as we can.