I get the impression it's a little like how Rolemaster handled it. You only ever play the one class, but you can borrow powers and skills from other classes to take your particular character a little off-road, so to speak. So there won't be fighter/wizards or cleric/rangers, but there will be fighters with some wizard tricks, and clerics with some ranger tricks, and neither of them will be the same as wizards with fighter tricks or rangers with cleric tricks.
So, based on the GTS scoop, it looks like you'll have to decide when converting a 3e multiclassed character over just how much he was class A and how much he was class B. Settle on one or the other, and then use the 4e rules to poach appropriate elements of the other class using feats (or whatever the method ends up being).
Think about it this way, too - so much of your character's baseline ability is purely level and ability score dependent. What does a class give you? Class skills to pick your trained skill from, and access to weapons and/or armor, and the powers. Mike Mearls has said that the game could, with a bit of work, become almost class-less, and I'm pretty sure that this would be the key to it. Single progression spiced up by choices each level of a power or a feat or something else. True20-flavored 4e?
Cheers,
Cam