Gaslaw asks on the forums whether backpacks will be bigger in Wrath of the Lich King, and (kind of surprisingly) Drysc says it's not beyond the realm of possibility.
I guess it's not that surprising-- it does seem like we are carrying more stuff around lately, in terms of quest and rep items, if not just plain old extra gear sets. We've already had lots of ideas on how to fix things, from creating more keychain setups, to just plain coming up with more bag space. But fortunately, Drysc doesn't seem to have just bigger bags in mind-- he seems to have "better stuff in the works."
What that could be, we have no idea. Pack mules? Dimensional suitcases (made, naturally, of sapient pearwood)? Finally, player housing? Drysc says we'll find out soon.