I would consult the pros at the John Bridge Tile Forums

That's something that's recently been talked about by the rep from Durock, both in the flooring forum, as well as John's Pros forum. I disagree. It shouldn't be put all the way to the membrane, but it SHOULD be embedded in the mud, if for no other reason than the fact that there would be nothing else to hold it in place, being you're not supposed to put nails or screws through the pan membrane lower than 6 INCHES above what's going to be your finished shower floor surface!

So long as your pan membrane is properly pitched, and you have gravel around the weepholes of the drain, no water will ever get to the bottom of the cement board.

Thanks for the replies Terriks and Bill. I'd searched the bathroom forum for help but didn't think to search the flooring forum. I'm learning a lot about this type work thanks to folks like you.

I can see the mechanical advantage of embedding the boards in the pan mud, very stable that way. My concern, of course, was water wicking up into the backer board. It seems that might still occur with the board embedded into the pan. Someone posted, here or in John's site, that he waterproofed the bottom of the boards before embedding them which would eliminate the wicking problem. But I'm thinking the moisture that could seep through the grout and into the backer board would migrate down and be trapped by that waterproofing. Would a coat of redguard or similar product along the bottom of the boards and along the pan edge AFTER embedding them into the pan be appropriate? It seems this would allow moisture to migrate down through the backer board into the pan and down to the pan liner to the weep holes in the drain. Am I over-analizing this? My goal is a well made shower that will last as long as I do.

Thanks again.

Someone posted, here or in John's site, that he waterproofed the bottom of the boards before embedding them which would eliminate the wicking problem.

That was Scott Crandall, the Durock rep. He's posted both in the flooring forum, as well as the pros hangout at John Bridge's. I disagree that if constructed properly, including keeping the bottom of the cement board off the pan membrane, that water will have the chance to wick up the wall, though. If you want, you could coat the bottoms of the cement board, but I don't think it's necessary. Quite frankly, if a customer asked me to do it, it would be considered an extra.