Just let people download(DL) (through bit torrent protocol if you want to save massive amounts of bandwith) the D3 client. Not just those who bought the 1yr WoW sub. or pre-paid the digital copy. [maybe even only beta testers, i dunno]
I'm in the beta, getting the collectors edition, but I don't play WoW. I shouldn't be punished by having a gods-aweful post-purchase install just because I don't play WoW anymore.
Hell you could just release the client on ThePirateBay and be content with it.