Might as well break out the full story since I know something about this.
Long story short, as Machamp said, several people were accused of stacking on camera. The main incident was during Top 4 when a player did something similar to double nickeling, which is essentially a shuffling technique that perfectly distributes every card in your deck. The other example was when the champion during one of his top cut matches (forgot which) declumped several cards in his deck and then didn't shuffle properly afterward. Declumping is technically legal as long as you throughly randomize your deck after doing so (which thereby makes declumping an idiotic procedure because if you shuffle enough afterwards, the declumping didn't matter in the first place).
Nothing has happened yet in terms of punishments. Personally, I think that if sufficient evidence is found (the whole thing is on video) that both players should recieve harsh punishments. Cheating is inexcusable in my book, and if you feel the need to cheat, you shouldn't be playing the game. Play the game for real rather than manipulating it to your advantage.
On another note, the judges also made some serious mistakes. This should have been caught IMMEDIATELY. There was also an incident in another top 4 match where a player used Ghetsis and his opponent discarded the items instead of shuffling them into his deck. This happened TWICE and NO ONE noticed, including the two judges who were watching. I don't mean to hate on the judges who clearly devoted a large amount of their time to help out at this event, but that's inexcusable considering it drastically changes the outcome of the game.