Originally Posted by Wobbuffet
They do spot checks throughout the day and once you get through to the finals all carts are checked. If he was using modified Pokemon he surely would have been caught?
Not really as the official legit checks are terrible (they rejected the event Weavile given out at Worlds the previous year, at last year's competition).
Previously the legit checkers would mostly check if the IVs and the PID matched. IV numbers where set to a certain method of coding (called the PID, I think that stands for Pokemon ID but I'm not sure). Also in Gen 4, the PID also connected the IVs to the nature. Think of it like the link that connects IVs to nature and (I think) ability. If this link was wrong then the Pokémon was not made in game, as the game could only make certain IV combinations with certain natures.
Now in Gen 5, they scraped/are not using the IV/PID combinations in the game making it harder to guarantee that a Pokémon is legit. My guess is that the staff are still using an old version of the legit software and 99% of them haven't a clue what's actually going on.
EDIT: Just crunched numbers on my VGC 2011 Battle. My Landourus's speed at lv50 is (IV 30 in speed and 255 Evs in speed) 167, yet I was outspeed by a Sigilyph with Life Orb that can only have a maximum speed of (max IVs, EVs and speed nature) of 163..... *sniff sniff* external editing