Not exactly true. What AT&T have told me is that they are not in the software development business, so they will not be developing the app. Therefore, all that needs to happen is a 3rd party developer needs to develop the MMS app - just like O2, T-Mobile, and Vodaphone have done. But, AT&T is not going to develop it and neither is Apple. Someone else has to step up to the plate and put out an app. Since other MMS app work-arounds have been approved lately, I see no problem with getting this through the approval process.
I would think there would have to be some sort of restriction in the US otherwise we would see the AppStore flooded with MMS apps.