Wonderboard is really quite difficult to work with, when it comes to trying to do siding. Getting a piece to have nice straight edges and making inside cuts with it to fit around windows and other penetrations is difficult if not impossible to do without it looking somewhat awful.
I doubt you would find a professional installer or contractor who would be willing to side your home with wonderboard, out of fear for liability issues down the road, (I know I wouldn't do it).
If you are really interested in a fire retardant siding, take a look at hardiplank, which is manufactured by the James Hardi Corporation, http://www.jameshardi.com. The issues you would have to deal with by siding your home with wonderboard would only be worse when you go to sell it. Finding a buyer who would like or trust this material as a siding is unlikely, in my opinion.