The new iPhone will be able to tether in the future, but I believe it will be around $70 bucks for tethering and possibly texting plans. That does not include the data plan, which is currently required for all iPhone users. The way around the tethering contract is to jailbreak you phone, which allows you to install any third party stuff you wish. I've done that on my 3G model, and can tether without AT&T knowing anything about it. So that's one way around that, once the 3.0 Jailbreak is released (supposedly tomorrow). Honestly, Jailbreaking is one of the best things you can do to your phone. If done properly AT&T doesn't know anything has changed, and you can add all the cool stuff you want to. ROM's, emulators, tethering, themes, wallpapers, OpenSSH access, the list is almost endless. Definitely well worth the extra effort, at least in my opinion.....
As for the MMS, AT&T is claiming that they have to add "Opt Out" options to all the accounts before they can roll out the MMS service. Sounds like a bunch of crap to me, but they say it'll all be ready to rock this summer. However, one of the local stores I talked to said they think it might be released as early as next month, as a "surprise" move. Don't know for sure though, so don't quote me on that. As for price, what I've heard is that it's going to be included in the current text plan, so there will be no additional charge. Apparently, even though you haven't had access to it, you've been paying for it from day one.