From what I've seen, the Normal/Nightmare/Hell/Inferno of Diablo 3 is Casual/Normal/Veteran/Elite in Torchlight, whereas the second playthrough only scales the levels of the enemies and doesn't necessarily raise the difficulty.
Oh, it appears you can "new game +" indefinitely, with the monsters capping out at level 120 while the player cap is only 100. Shit's gotta get tough there...