A feature I would really like to see in the BIAB for iPhone app is the ability to add a rendered song to your iPhone's iPod music library. This would allow you to play a rendered song outside of the BIAB app which would be quite useful. For instance, it would allow you to play a song in the background while you used a score app. Or even better, you could use apps like "Capo" (a music practice app) that tap into the iPhone's music library.

Of course you can add BIAB songs to your iPhone by adding them to iTunes and then syncing your iPhone, but it would be much more convenient if added to the BIAB app instead.

I don't know if this is technically feasible, but other apps that take photos or video can export to the iPhone's standard camera roll, so I can't see why it can't be done for music too.