I'm not seeing (in my minds' eye) any benefit to doing the drum part on screen.  I think an MPD or a regular computer keyboard even would be so much cheaper and reliable to use.  I'm not sure sure at all about the level of noncomfortablicityness.
 
I've played with using my iPhone 4, using AC-7 Core HD using my iPhone for remote controlling of my DAW but it's a pain.. I've thought about grabbing an iPad to use for that but I could probably set up my Rocktron Midimate and have it work much more conveniently.  It's a very cool app though.