If you REALLY want to make money in Cata... =]
Start stocking up on pre-Cata mats (from any of the WoW-eras [classic, BC, and wrath]), and sell them in bulk post-Cata release. OMG I just had a braingasm.
Somewhere a gnome drops a vile of Arthas' undead plague and restarts the Zombie Event from the opening of WotLK.
That should get a couple of weeks/month of fun city raiding, and tears from people saying it ruining their playing experience...
Is the answer to the Question...