My 2.5 cents, take it or leave it:
I think this is a really cool idea and an awesome toy to have in the studio, and I totally have huge respect for you developing this and making it work as well as I can see that it does. However, I can't really see this being used as a serious mixing tool in a professional studio. You say its a program for professional users, but from my experience, professional users are already going to have control surfaces that they prefer in their studios, or have developed ways of mixing with the mouse, that are faster and easier than using the (comparatively) very small touch screen on the iPhone for their mixing needs. I'll admit my experience is limited, I am young, and I might be completely wrong about this, but thats how I see it.
The way I see it, this product is more of a novelty product than a serious mixing tool, and I think that by pricing it for professional users, you are eliminating a lot of your potential market for this app. If it were priced more like the majority of other apps in the iTunes store, I think you would get a lot more people to buy it simply to try, and to have around for the few times it will be used than if you keep it at the "professional" price point its at now.
Just my humble opinion though, best of luck to you!
-Mike