Been around for a little bit:
http://line6.com/midimobilizer/... hasn't gotten stellar reviews, but I think that's because they're asking quite a bit of the phone (what it was originally designed for) to both control instruments and/or play digital samples as well...
There're some control surface apps for the iPad already as well - virtual mixers and such, which are *VERY* cool. I use a virtual device but mix all on the Mac Pro - the iPad-as-control-surface would look just really cool in the gear bag...
