I downloaded the AC-7 Pro Control Surface for the iPad, and took it for a spin. Installation was actually about as difficult as some people have suggested in the comments on iTunes; i.e. it took me about half an hour to get things working. It took some messing around and a few tries, but I think I've got it working pretty well with Final Cut now.
You have to follow the instructions on the SETTING UP page of the developers website. Specifically, for Mac users, you have to install DSMI Server for OSX and set that running. Launch AC-7 Pro, and then launch Final Cut Pro, and go to Tools>Control Surfaces... and add a Mackie control surface. It will default to DSMI Server as the input and output. And then with a little luck, you're off!
I'm still working on the order for launching the AC-7 Pro, because I find that it doesn't always seem to communicate both ways with Final Cut; i.e. the iPad makes Final Cut's sliders work, but if you playback a mix on Final Cut, the AC-7 Pro's sliders don't move. Quitting and relaunching the AC-7 Pro app usually seems to fix that problem.
It's really pretty cool, having the sliders move up and down on Final Cut as you move then on the iPad; and the other way around as well!
Play, Stop and the sliders work. The FF and REW buttons sort of work; FF seems to just Play, while REW plays in reverse; I wish they jumped to the beginning or the ending of a segment. The Jog dial works though, so that's pretty neat.