Maybe I should clarify: with respect to design decisions, they're pretending dual spec does not exist. Raids and talent trees will be balanced without reference to the fact that some players will be able to easily swap out. GC gives some specific examples of things the devs will not be doing, such as "Have a fight that requires 8 tanks, because you know, half the raid can change specs," or "Let Arms be the PvP tree and Fury be the PvE tree, because warriors can just switch out."
This is good news to many, including myself. Although if I had been asked to guess, I would probably have predicted they would take this direction, it's good to know that (for instance) raiders won't be effectively required to take spec X and spec Y in order to complete encounter Z. Dual specs are there to offer us more flexibility, not as another way the devs can make us jump through hoops. And I know I'll be taking it on at least a few of my characters.
Gallery: Patch 3.1 Dual Spec walkthrough
Patch 3.1 brings us Ulduar, dual specs, significant changes to all the classes, and more! We've got you covered from top to bottom with our Guide to Patch 3.1.