Yeah, I agree with Shuli. I think it has a lot to do with personality and parenting. When I meet adult twins I often ask them if they're close with their twin. From the ones who aren't, I've heard more than a few times that they felt resentful that their parents compared them to each other, often negatively, such as, "Your sister always does X and you don't!" kind of thing.
My fraternal girls are *very* close and bonded. Obviously, I don't know how it is with identical twins, so maybe they're closer than my girls. They're only 5, though, and we haven't done preschool so they've had more limited opportunities to form relationships separately with other kids. We'll see what happens when they start kindy this fall.
SAHM to F & P, : fraternal twins born 3/05, : I, born 12/07 & at 5 weeks in July 2009