Maybe I'm just weird.
Perhaps it's not technically cheating, but that's how I classify purposefully viewing another player's cards and using that information against him/others. Doesn't seem like a moral gray issue, just a total scumbag move.
Let's say you're in a hand and are attempting to bluff a pot representing the nut flush, but your opponent had already seen his neighbor's hand which contained the nut flush ace and uses that information to beat you. You guys wouldn't consider that essentially cheating?