I heard about a product on a home improvement show that you paint over Durock before tiling. This paint becomes a "rubber" surface totally sealing out any moisture. I suppose any moisture that may get behind the tile would drain down the the top of the tub.

Does anyone know what this product is or if it is even necessary? The guy at my local paint store (that also sells tile) told me that you want the cement board to absorb the moisture and then it evaporates. I'm not sure where it evaporates to if it's trapped in the wall but that's what he said.

We're going to install 12" x 12" porcelain tiles set on a diagonal.

I appreciate any comments.

Yes I have used this product, my tile supplier sold it to me. He had a demo on the counter. A filled carboard box with water. I forgot the name of the product. But his material painted over virgin material will form a rubber seal that will not leak or allow vapor to pass through. Although I still used a rubber sheet below my mud floor in the shower we were building. I gave it a try. What I did not like about the product was they required me to use their cement to hold the tile on the wall. I suppose to assure that there was nothing in the mix to cause issues with the paint. But regardless of what material you use, the drying time is much longer as the moisture is not wicked out by the tile board, so the cement stays wet longer. Both good and bad. Good is that it drys longer and sets stronger, Bad is that it takes longer before you can properly grout the wall.
This is a fairly new product, past seven years or so. I would stick to the old tried and true method of vapor barrier, tile board, we use Dens Armor Plus instead with fiberglass tape and the special mud they sell. Stainless screws. Then tile the wall with a good portland tile cement.