As Oz said. You can go to
http://www.worldofwarcraft.com and login to your account (top left in the account section) and if you scroll down there's a Download link. It will download the current version (minus maybe a patch or two which it will just get after install) for you.
That's what I did. I have WoW on my PC (my gaming system), and decided to put it on my new MBP to see how it performed (amazing BTW) and the installer took care of everything. Pretty fast too... was 900+ KBps download the entire time.
Though, regardless, I'd recommend to save your "addons" folder if you've added anything at all to the installation (cartographer, quest helper, Auctioneer, etc) and put it back in the folder after your installation. The default addons folder is World of Warcraft/Interface/Addons.
D.