However, seeing that necessity is the mother of invention,...
...my bro-in-law Kevin came up with this ingenious method of transporting/storing multiple Dominion sets.
Check it out: three-ring, zip-up binder briefcase full of plastic card-holder pages. The cards are organized first by expansion set, then alphabetically. Here's a page from Intrigue:
At the front of the briefcase is this pencil pouch (which I literally used in fourth grade), that holds all the little trinkets, like victory and treasure tokens.
Those toothbrush travel cases contain tokens. Kevin's full of inventions.
Voila: a convenient carrying case that currently holds four Dominion sets. Perfect for taking to the park to play on a blanket, in the car on a road trip, or just to a friend's house.
I kind of want one now.
Update: We made a new box for our cards and the DIY can be found here.