The only decent Bible app available is YouVersion, which does not allow downloaded bibles and requires a data/wifi connection. If Laridian or Olive Tree would do one, they could corner the market, at least for now. Links to suggest this for Laridian and Olive Tree are below:
http://blog.laridian.com/?p=766&cpage=1#comment-7528
Select a Device -- Olive Tree Support
Try LDS Gospel Library, it can even read the scriptures to you.
http://www.windowsphone.com/s?appid=d0d42f75-e171-e011-81d2-78e7d1fa76f8