As far as the trinity goes, it's pretty much always going to exist to some extent. In Guild Wars, yeah you didn't need a tank for most content because of how effective protection spells and monks in general were. However, you were pretty much required to use them if you had any hopes of beating the more difficult, elite areas without a huge hassle. Obviously, if one is skilled enough, you could, in theory, do all of the hardest areas in the game without a tank, but it's significantly more difficult.
I figure the same will hold true for GW2. It's not outlandish to think that the more difficult content will have a Warrior, making use of his unusually high VIM stat, soaking up the majority of the hits, while you have some water eles and guardians supporting him and keeping the group alive, while some people in the backlines provide DPS. However, one of the fundamental differences between this set-up and ones found in other MMO's will be that if the tank goes down, in THEORY, someone should be able to change the way they are playing to control the mobs themselves, without getting owned in the face. How this plays out, though, we'll have to wait and see.