Yes, they probably will ship 50 mill by des 31 2010, considering they've shipped 39mill as of now. Will they sell 50 mill? Probably not, but it'll be close.
Why do I think so?
-For the simple reason that its going to be a strong software year for x360.
-They will cut price
-They will relaunch the system when NATAL launches, similarily to what Sony did last year.
-They will subsidise and launch a massive marketing campaign of epic prportions.
MS is in no way out of the game. That said, Sony will probably still be around 4-6mill behind x360 at the end of des 2010. Nintendo will still have a massive lead over both.