There are several libraries for Android that allow using OSM, displaying OSM-based maps, including at least:
* [osmdroid][1] ([Google code][2], similar to Android's MapView)
* [Mapsforge][3] ([Google code][4], offline vector rendering)
In addition to that, the [Android wiki page][5] has a full list of OSM software running on Android. Android, including not just map viewers, but also routing and editing applications. A lot of that is these are Open Source, so code sharing/reuse would be a possibility.
[1]: http://wiki.openstreetmap.org/wiki/Osmdroid
[2]: http://code.google.com/p/osmdroid/
[3]: http://wiki.openstreetmap.org/wiki/Mapsforge
[4]: http://code.google.com/p/mapsforge/
[5]: http://wiki.openstreetmap.org/wiki/Android