It'll never happen. AT&T is a company, just like Apple, and will charge you for anything at all. I honestly wouldn't be surprised to find that, when 3.0 comes out, they implement a max number of MMS message you can send on your SMS plan (even unlimited SMS message plans will probably have a limit), and will end up charging an additional amount if you want unlimited MMS messages, too. It's smart business really; it keeps people from overloading their network as most won't pay an extra premium for a feature they don't use all that often, and those that do need it will generate more revenue. Hope I'm wrong, but if I were in their shoes, I'd do that.
The same goes for tethering. No way they will allow tethering without an additional cost. Tethering WILL bog down the network if anyone can do it, and if you're using your computer frequently by way of your phone, that 5GB cap is going to become useless pretty fast. Let them charge an extra premium so you can have unlimited data usage when tethering, and again, keep the networks semi-clear from people who won't use it unless they NEED to be paying for it.
Of course, the big picture strategy would be to fix the network altogether so MMS, Tethering, and SlingBox don't bog down the network.