This article has a summary of the discussion the VoiceCentral app developer had with an Apple representative. It basically comes down to this:

Apple - Your app (that we already approved) duplicates iPhone functionality and is being pulled.
VC Dev - "Can you tell me what portions of the app were duplicate features?"
Apple - "I can’t go into granular detail."

Completely black box. Apple really needs to take a look at how approval processes work for other closed systems like game consoles and mirror them. It would really help improve these types of situations for both parties. Like it or not, the success of the iPhone partially relies on the developers making applications for the platform. If Apple manages to drive major players away to other platforms, it will harm them long term. Even my mother on her iPhone uses 3rd party apps all the time.