What wall material should be used behind the Kerdi system in a tub/shower?
Schluter says that behind the Kerdi membrane a solid backing needs to be used.  They define a solid backing as :


gypsum wallboard
cementitious backer unit
fibercement underlayment
fiber-reinforced water-resistant
gypsum
backerboard/underlayment
coated glass mat water-resistant
gypsum
backerboard
portland cement mortar bed
concrete
masonry


Found this in their Shower System Installation Handbook here.
Link : http://www.schluter.com/media/ShowerHandbook.pdf