While not stash space... they pretty much did sell it. It's called the collectors edition. 3 extra mule *cough* character slots. Which is about the only thing you really get from the collectors edition, really.. Unless random aesthetic-only pets for games that you may not even own appeals to you.
I can see some sort of microtransaction for Diablo 3 in the Blizzard Store in the future. It's a B2p game, and common practice in the gaming industry is to monetize a game after its release too. Since Blizzard isn't seeling us DLCs each month (hi Borderlands 2), it wouldn't surprise me to see them selling us storage tabs, character slots and cosmetics in the store.