Well, I think I finally figured out what is happening. It looks like the obb file in question was already mounted. The file was an APK expansion file and it would appear this is automatically mounted for you by the system.
If, before trying to mount it, I execute:
if (storageManager.isObbMounted( obbFile.getAbsolutePath() ))
{
Log.d("", "obb file mounted at " + storageManager.getMountedObbPath( obbFile.getAbsolutePath() ));
}
without first explicitly mounting it, I get a valid mount path. I really wish the docs (or LogCat messages) were more explicit about this. It took me about a day to discover this. It wasn't until I explicitly used an (necessary) encryption key on the obb that I saw a message in LogCat saying the file was already mounted.