From the '06 Rules by Topic...pages 68, 69
 

It is a legal forward pass, if during a scrimmage down and before team possession has changed, a player of A throws the ball with both feet of the passer in or behind the neutral zone when the ball is released. p. 68

Situation C deals with a punt muffed by R beyond the neutral zoen and recovered by K behid the neutral zone. K threw a legal forward pass after recovering the muffed kick. It's not exactly the same play, but the all did cross the NZ before the legal pass. p. 69

In Simple Terms: The passer may cross the line of scrimmage, retreat behind the line and throw a pass as long as both feet are behind the line when the pass is thorwn. p. 69