Dausuul said:1 in 3,037, actually (rounded to nearest integer).
That said, if somebody seriously accused me of roll-cheating purely on the basis of rolling six 20s in a night, I'd walk out of that game and not come back. Coincidences like that happen all the time. The odds get a lot shorter once you spread them out over all the players in the group--in all the groups--with the kind of play schedule they undoubtedly have going on at WotC right now.
He wasn't reporting on all groups, though. He was reporting on 1 group that hasn't met for 2 months. Even with 6 players making 20 attack rolls apiece, at that rate it's more like 80 years before you'd see such an event.
Now, if I were actually playing with a person who did that, I wouldn't accuse them of cheating anyway, unless it happened too often and other players were getting pissed. I bring it up here because this is one of the behaviors playtesters should measure.