Since the iPhone came out, I've been wanting a composition sketchpad app to carry around with me, but the notation apps that have appeared so far are mostly toys and lacking enough functionality to be useful to me. Besides, the iPhone screen is just too small to work on comfortably. Now that the iPad has appeared, however, the game has changed. I started using Encore on a 1024x768 monitor years and years ago - back when that resolution was considered huge - so I know it will work amazingly well, especially in the portrait orientation. Since GVOX already has the most intuitive user interface out there with Encore and MusicTime, it would seem like a natural for them to get the iOS SDK and get after it.
What would make the most sense to me for a mobile touchscreen device would be a minimalist interface without any of the extraneous palettes - Dynamics, Expressions, Marks 1, Marks 2, Guitar, Graphics, Tools, Symbols, Color - and just the necessary ones to create the actual notation - Notes and Clefs. Key and time signature setup and changes would be necessary for me, because I change those a lot in the corse of longer pieces.
Then, just allow single and dual staff systems without any fancy hiding, and sizing, and limit scores to 8-12 systems max (12 staves with systems determined by how many were dual stave systems would work). Perhaps have it auto space only, and also only at engraver's spacing.
Remember, this is a mobile sketchpad for jotting down ideas on the run, not creating finished scores, so just a few native sounds like piano, guitar, bass, and drums would be enough. Then, have the files export in the .ENC format so that they can be imported into Encore, and work on them can continue with Encore's full features.
Somebody is going to do this, and I'd prefer it was GVOX.