I think we may see multi-classing hamstringed in 4e for this reason.
suppose you have a wizard/rogue... is he a controller? A striker? Both? 3/4 controller, 1/4 striker? 1/2 controller, 1/2 striker? or perhaps somehow 3/4 controller 3/4 striker?
It sounds like their answer in this edition isn't to find some elegant, balanced way of multiclassing,(which, with the multitude of classes becomes less possible) but creating new classes that fill the multiclasser's niche while still maintaining that party role.
So instead of having Wizard/Rogues, we might get a class called "arcane sneak" to be a rogue who uses arcane magic to fulfill his role as a striker, and a "shadowmage" to be a wizard who supplements his controlling type abilities with a rogue's subtleties.
meanwhile multiclassing will be just a way to tweak your character. If Saga is any indication A wizard/rogue will not be 50/50... probably more like 75/25.