Is it a draw by insufficient material if the board is K vs K, N & N? It isn't possible to force checkmate with King vs. King, Knight &amp; Knight. A checkmate position, however, is still possible to achieve, so does that mean the 50 move rule applies, or does insufficient material mean insufficient material to force checkmate?
This is a complicated situation, and it is made even more complicated by the fact that USCF and FIDE rules differ slightly.  

First and most importantly, the 50 move rule always applies.  If 50 moves are made without a capture or a pawn move, then the game is simply declared drawn.  

The rules about insufficient material only are important when one side runs out of time.  Basically, the rule says that if a player runs out of time, and the opponent does not have enough material to mate, then the game is declared drawn instead of the side with time remaining getting the win.  For example, if black has a lone king and white flags while trying to mate with a king and queen, the game is a draw.  If there is insufficient material to deliver mate, an OTB game can also be declared drawn even with time on the clock if a tournament director is summoned.  This will only apply for cases like K vs K or KN vs K.  Usually it is not an issue because the opponent will simply agree to a draw.

So now to the specific case of KNN vs K:

FIDE

Under FIDE rules for standard tournament time controls, if the side with just a king runs out of time, then the side with KNN wins because a mating position can be constructed.  If the side with just the king avoids one move mates, however, then mate cannot be forced.  

USCF

Under USCF rules, if the side with just a king runs out of time, then the game is declared drawn.  The USCF focuses on the fact that checkmate cannot be forced.

Online Play

Many online chess servers will automatically stop a game when there is not enough material left on the board to mate.  In the specific case of KNN vs K most servers do not automatically give a draw (following the FIDE rules).  One interesting case is KN vs KB when Playchess and ICC will stop the game and award a draw (not following FIDE rules) while Chesscube follows the FIDE rules and continues play.  

Practical Play

Now for practical play, if you are defending with just the king, it is very easy to avoid mate for 50 moves, even if you have almost no time remaining.  Make sure that you don't move to a square where mate can be delivered next move and you will never be forced into mate.