From what I can remember from my undergraduate days, it seemed like it took very little effort to get by, i.e. to maintain about a 3.0. Let's call this "X" amount of effort.
To get a 3.3 probably took about 1.5 X amount of effort. To get a 3.6 probably took about 2.5 X amount of effort. To get above a 3.8 probably took about 5 X amount of effort.
Going into b-school I imagine this sort of effort-return relationship isn't radically different. With networking and all the other important non-academic stuff going on, I'm wondering how hard everyone is planning on studying. For those going into banking and consulting grades will be important (whether or not your school has grade disclosure, from what I've heard they will look for distinctions or honors or whatever from NGD schools), but still nobody wants to spend all their time studying to get a 4.0 do they?