I'm waiting for a "PARTIAL BIAB" app for my iPad which includes (only that part is already worthwhile) a transpose function for sheetmusic. In fact I don't understand why BIAB didn't develope it since it will boost the use of BIAB in general enormously. To explain what I mean is that me and many many users with me, mostly use the BIAB program to edit, print and tranpose music before printing but now primarly printing to pdf files for iPad (nowadays instead of paper). Untill now biab sheet music was used by all my band members (3 bands) I know and see the same on many jam sessions. However nowadays everybody starts using iPads or alike and read music from pdf files (books)!
As known the "iReal b" app is used by many to transpose music on the spot. However it only contains chords. That may be sufficient for many musicians but many others like to have the presence of some music notes as well. The standard BIAB program has this transpose function already built in. So just an app which is able to use BIAB files and show the file at iPad. It cannot be a difficult task for a BIAB programmer as long as you don't try to get your whole BIAB program with all the functions into the app. Just such a partial program can be sold for good money and it will boost the use of BIAB in general. So what is delaying you?? And by the way "on the spot" means without the necessity of a wifi or 3G connection!
Since 2 month I use the iGigBook reader containing now already (in my case) 67 books. Very nice since on any occasion when somebody comes up with a number the music is directly available for me. Only of course as long it will be played in the original key. Almost on any occasion one wants to play the music many times a different key due to the presence of a singer. So if you make such a transpose function available everyone wants to use BIAB