For 3 years I worked on a startup that built SMS applications. Although we did not work with MMS, I am very familiar with the struggles regarding carriers and just getting set up. Ultimately our startup failed - our costs were just too high, our slice of the pie was too small, and to reach the volume that we would need to become self-sufficient would require a bigger financial investment in advertising than our CEO was able to acquire or willing to make. Anyway....
I read over a long email string that I had with reps from several carriers regarding setting up direct relationships with them. At the time, our company was working with the carrier aggregator (middle-man) mQube - they were then the best, but we wanted to cut them out of the picture. If you don't already know this, carrier aggregators are essentially the gatekeepers to the carriers, and if you want to do ANYTHING substantial, you need to go through an aggregator. The alternative is to have direct-binds with the carriers themselves using lease-lines or special relationships. Through my correspondence, direct-binds are no longer offered, therefore it is pretty much impossible to become an mQube (Verisign became an mQube late in the game only by purchasing mQube). The next problem is that mQube's UI and customer service is pretty horrible, and you have to go through it if you want to get your program brief approved. Getting your program brief approved will most likely be your biggest challenge, and program briefs are like $1000/mo at least. The different carriers have different requirements, etc. You'll probably have to peruse and follow something like this: http://www.mmaglobal.com/bestpractices.pdf
We spent months trying to convince mQube that we weren't going to spam people and that our app was legit. We were a non-subscription based service that supplied information/content only when a user initially requested it. That type of application is in the VAST minority of SMS apps, as most out there are horoscope type subscription services that people have trouble opting out of. In short, you are always going to be playing the carriers' game, and you will probably never win or even break even unless you have BIG BUCKS.
In Europe and Japan, if I remember, and it may have changed, there is only one cell provider ie Vodaphone, and it is much easier to get things done. The CTIA has the potential to create some standardization, but I don't see that happening.
Best of luck