1/2" concrete board, then Kerdi = waterproof. Some guys use denshield or hardi as the backer and I've talked to one contractor here that uses easy board - then kerdi (can't remember who!). Schluter kerdi can even be installed on normal drywall - afterall it is 100% waterproof, however, I tried it and went back to CBU.
Point is, if you are doing a shower or tub surround you are, in my humble opinion, not doing your customer any favors by NOT using either kerdi or, if you prefer, one of the other aformentioned liquid membranes.
And please don't tell me people won't pay extra for something as silly as waterproofing. EVERY customer of mine paid for and got a waterproof tile job (complete with epoxy grout). I can't think of one that thought that was a bad decision.