Both sides vulnerable.  
East deals.
                              North
                              S: 7 6 2            
                              H: 4 2         
                              D: 10 8 3      
                              C: A J 9 4 2
            West                             East
            S: 10 9 5 4 3                    S: J 8        
            H: 8                             H: 10 7       
            D: K 9 4                         D: A Q J 6 5 2       
            C: K Q 8 7                       C: 10 6 5      
                              South                         
                              S: A K Q
                              H: A K Q J 9 6 5 3
                              D: 7         
                              C: 3        
          The Bidding:  
                   East   South     West   North                  
                    2D     4NT      Pass    5D  
                   Pass    6H    (All Pass)
          Opening lead: KD
     At a duplicate game in Hilton Head, East opened two diamonds
and South had a problem.  He decided that all he really wanted to
know about was whether or not North held an ace or two.  4NT
seemed to be the way to get that information.  West asked North
what 4NT meant.  This gave North the chance to say "Blackwood"
and confirm that he was showing aces.  It is best, as the player
holding the West cards, to not ask this question.  North may not
pick up on the fact that South is bidding Blackwood and who knows
what could happen.
     The point is, when 4NT comes out of nowhere, it is Blackwood
and you should answer appropriately.  Think of "Occam's Razor".
When faced with any problem, the simplest solution is usually the
correct solution.
     When the results where in, two pairs stopped in game and one
pair stopped in a partial.  I think that if East had passed,
South should open 4NT.  Any other bid could lead to many
problems.