According to the current FIDE rules of chess:
The game is drawn upon a correct claim by the player having the move, when the same position, for at least the third time (not necessarily by a repetition of moves):
a. is about to appear, if he first writes his move on his scoresheet and declares to the arbiter his intention to make this move, or
b. has just appeared, and the player claiming the draw has the move.
Watching the current World Cup an interesting episode appeared when Wang Hao claimed a draw against Alexev Dreev. Wang stopped the clock and contacted the arbiter, claiming the draw. The arbiter saw that Wang had not written down his move before claiming the draw. Therefore Dreev got 3 mintues extra time and the clocks were restarted. With only seconds left on his clock Wang wrote down the move he planned to make and stopped the clock again.
Crazy as it might sound, if it now turned out that the position had not been reached for the third time the arbiter would have had to give the game to Dreev (as you apparently can only claim something incorrectly once). Luckily for Wang the position did appear for the third time and the game was drawn.
According to live commentator Susan Polgar the rules for claiming the draw have changed several times and are different from federation to federation. I certainly understand Wang as in any other situation writing down the move before you make it is not legal according to the same rules. My brother claimed a draw in England some years ago and then the arbiter told him that he should not have written down his move before he claimed the draw (his GM opponent then deviated and avoided the draw).
Finally my question(s): When did the rules change? What does the USCF/BCF rules say? Why does not the federations follow FIDE on this? Do you know of any other rules?