Lubomir Kavalek explains this act of sportsmanship.
The World Cup's knockout format can lead to some rather random results, which would have been particularly unfortunate given that this event is being used to qualify three players for the World Championship. But the four quarterfinalists are all among the world's elite: Grischuk faces Ivanchuk, and Svidler faces Ponomariov. The winners automatically qualify for the 2014 World Championship cycle; the losers play a match, the winner of which also qualifies.
Someone had to lose the match between fan favorites Gata Kamsky and Peter Svidler. Kamsky had the honor of losing to the most beautiful move of the World Cup:
Svidler's point after 26...Re2!! was that 27.Qxe2 loses to 27...Qg3! (The immediate 26...Qg3 is met by 27.Nc6, stopping the mate threat.) Someone (I don't remember who) described the combination as worthy of Frank Marshall. Edit a few days later—aha, it was Mig.
Early risers are urged to check out Game 1 of the semifinals tomorrow morning at the excellent tournament website! If you hear Russian, don't panic: there is an English-language simulcast.