Joking aside, obviously Wryxian says Ulduar will be harder, but it's not easy to put a number on difficulty. He says "it has to be experienced to understand." Blizzard has already said that they want to change things like mana regeneration to be less frantic and more strategic, so it'll be interesting to see just what kinds of mechanics appear in Ulduar -- odds are the difficulty probably won't be of the "Bosses that hit really, really hard" variety, but rather that we'll see more situations where the raid's attention is divided x number of ways. Maybe 64?
At any rate, the PTR should be up soon, if not tonight considering all of the reveals Blizzard has done this week, and then we'll have a better idea of what the next raid has in store for us.