Well, if you play as a fury warrior you can utilize the Titan's Grip talent (will be converted to a passive ability in Mists of Pandaria) to wield two two-handed weapons at once. If you are just into showing off some big weapons, you could purchase two copies of
Grand Marshal's Claymore very cheap and run around. Unfortunately, as far as I know, you cannot transmogrify that item so it's not viable for any end-game raids or dungeons. Though there are plenty of other big swords you can transmogrify, I think the largest in-game sword is
Armageddon.
Also, in Mists of Pandaria Fury-warriors will be able to use Bladestorm, so if you have two weapons equipped you'll look kinda cool:
http://www.youtube.com/watch?v=tbjDl4TWKsE