I received an e-mail from the creator of SeeScore (AppStore Link), who informed me that SeeScore is available for inclusion in other apps wishing to take advantage of MusicXML files (without having to recreate the code).
SeeScore is a unique app that takes advantage of MusicXML files, converting them into notation–and allowing free-flowing resizing of musical text, the ability to add or remove included staves, and playback. It's the only app I know of that has these features. NeoScores is bringing a similar service to HTML 5.
The only negatives are connected to MusicXML itself (which I love as a format). The first is the relative lack of MusicXML scores (many times, you must create them yourself), the other is the inability to make persistent written notation over MusicXML text (i.e. lack of annotation).
So, check out SeeScore, and if you are a developer would would like to include SeeScore in your apps, check out their developer page and send them an e-mail!
Note: there is a free “lite” version of SeeScore available, too.