Is it legal for a player to take a throw-in from his knees? Where is this specified in the FIFA Laws of the Game?

Answer (April 11, 2013):
It is not included in the Laws of the Game. Outside the Laws of the Game, we are aware of only one document that FIFA has issued for the IFAB (the people who write the Laws) that covers this situation. It is in Law 15, Q&A 7, of the 2006 edition of Questions and Answers:

7. Is a player allowed to take a throw-in kneeling or sitting down?

No. A throw-in is only permitted if the correct procedures in the Laws of the Game are followed.

From the USSF Advice to Referees:

Players are not allowed to take throw-ins while kneeling or sitting down. Squatting is a form of sitting and is therefore not allowed, but players are permitted to take “flip” or “acrobatic” throw-ins, provided the procedures outlined in Law 15 are followed. “Standing” is the normal and traditional posture at any restart; anything other than standing is not permitted. The “acrobatic” or “flip” throw-in is allowed because the thrower actually makes the throw from a standing position.

NOTE: The kneeling answer was also in earlier editions of the Q&A, which is no longer published, having been replaced by “Interpretation of the Laws of the Game and Guidelines for Referees ” Simply because this fact no longer appears in the Laws of the Game does not mean that it is not valid. Many items not written in the Laws are widely known to be valid. The most famous omission is that the player who has been sent off during the game may not be replaced. Why these omissions? Because “everyone knows that!”