I swear I came up with this idea yesterday -- my hard drive has been having problems with World of Warcraft, and the thought occurred to me to buy an external USB hard drive and get WoW and all of my addons loaded on it. That way, I could not only avoid the errors on my current hard drive, but I could also take my installation with me whereever I went. I wasn't sure it would work (and after a few repairs and a reinstallation, I appear to have WoW working again), so I haven't followed it up.
But a company called Transcend was way ahead of me. In Asia, they're selling a hard drive preloaded with a working Wrath installation on it. So all you need to do to play Warcraft on any computer with a USB hookup is plug in and run. You'd likely have to adjust the graphics settings depending on which computer you plugged into, but it's a great idea. Available sizes run from 160gb to 500gb, so I'd assume as well that you can use the rest of the drive as normal.
It looks official as well, but you never know -- it could be the The9, the company that runs Warcraft in Asia, has approved it, or Transcend could just be doing this on their own. Either way, this wouldn't be too hard to make yourself here in the states -- buy an HD, install WoW, and there you go.