Well there was that guy who got gladiator 4 boxing shamans. In general shaman tend to be a popular class to multibox. They are fairly easy to control in groups since they don't require you to manage CC (earthbind isn't much of worry about, set it and forget it). The ability to have essentially endless interrupts is also one of the appeals of multiboxing shamans. They really become quite powerful as they get more and skills too.
Currently I've tried duel boxing mages and three boxing shamans, and both were actually relatively easy and straight forward. I can't imagine every class being as easy. I plan on taking my 3 shammy up to 39 and giving it a try as elemental to see how it goes and then I'll probably play with the two mages at some point.
For anyone interested in multiboxing you don't need any fancy set up like the picture linked early. My $1200 imac has no problem running 3 WoW accounts and the key cloning software was totally free. It was almost as simple as just making duplicate copies of WoW on my computer. It's fun for those who like technical problems or just to try something different.