Though most probably know (I didn't) you can download Diablo 3 now. Go to your BattleNet account and you will see a link to Diablo 3 and the installer.
It's 7.6GB, so best get it done early
Don't be afraid of cancelling it. I've tried it. I downloaded 3.2GB over night and cancelled it just now. Restarted it and all is well. Just need the other 4.4GB now