[bumblebee]

All this is very well, but has anyone considered where and how the source material is going to be acquired?  Seems to me this is the major hurdle that has to be jumped.

For myself, I have my own charts in Sibelius, but yes Graeme, I agree, there is not much available where there could be.

Just an additional comment on Finale Songbook and Sibelius's Scorch demises—the user base is large for both; many would have their own files and music publishers (and engravers) use either application for layout (before printing to paper) so there would be vast numbers of files in existence and yet the apps have failed (at least so far). Perhaps the new version of Sibelius will arrive with an update of Scorch for iPad … and a release for Android.
Orchestral (etc.) music does not even need to be transposed (usually) so its even more simpler to display the notation without any problems. A dedicated app supported by publishers (perhaps incorporating time-limited PDFs for leasing show music etc. where necessary), suitably bookmarked for orchestral works should work I would have thought. I suppose what many are waiting for is Hal Leonard and Sher Music to put out their own apps with their real book versions as in-app purchases. Why this has not been done already you have to wonder. They could either have a transpose function if they use notation files or just transpose into all 12 keys as an option—all saved as vector PDF so the file size is not huge. Doing the latter, they can manually tidy up the enharmonics, accommodate lyrics (as an option) and format specifically for tablets (no cropping needed with small margins, appropriate size and page turns) and where you could purchase individual songs in whatever key or purchase complete sets. As far as I know, this is not possible yet, although Sher Music has their New Real Book vol 1 as a PDF (for C, Bb and Eb instruments) but not transposable to any key.

Any musicXML app would have a much smaller user base than either Finale and Sibelius and at the very cheap prices typically expected for device apps, I am not sure they have the income for developing them. Looking at some of the musicXML apps, some have not been updated for a year or two and still lack many features. So if the big companies cannot sustain it, it is not surprising the smaller development companies struggle, if that is what is happening. 

MuseScore is increasing substantially in user numbers and their enthusiasm and dedication (both users and developers) make it more likely (let's hope) their app will fill the void.

Well at least we all have MobileSheets, thank you Mike for your time, effort and dedication.