I settled on Hardibacker 1/2 inch since it was supposed to be the lightest weight cement board on the market.  But those suckers are still really heavy.  I actually had to have hubs help me hang them.  A couple quick tips working with cement board.    
1. NEVER use green board, drywall or any sort of wood as a substrate for any area of your bathroom that will be wet.  Cement board or poured cement is the only thing you should using.  You can use other products in "damp" areas (like the rest of the bathroom) but never use them anywhere there will be direct contact with water. 
2. You need to score and snap cement board. You should not use any sort of saw on it since that will kick up toxic dust, not to mention chew through any sort of saw blade. To score it you ned a very sharp utility knife and you want to just keep going over and over the cut line until you can easily break the board cleanly.  It is NOT anywhere as easy as snapping drywall. 
3. When attaching cement board to the walls you must use special screws designed only for cement board.  The hardibacker required a screw type that actually had a square head.  In the box of screws they also provided a drill bit for use with the square heads.  I needed my power driver to get the screws to lie flush with the cement board, you would not be able to drive those screws in by hand.  They should be into the studs, no more than 7 inches apart. 
4. 1/2 inch Hardibacker is NOT actually 1/2 thick.  If you need to abut it to 1/2 drywall it will not be flush.  You need to shim behind it, on the studs, to make the wall flush.  You will eventually tape and mud the seams and also have a chance to feather out the difference in height, but a smaller gap is always better. 
5. When installing cement board you want to leave at least 1/8 inch seams between the boards (for expansion if they ever get wet.)  Getting super accurate cuts was pretty hard for me, so leaving the gap was no problem. 
6. Make sure you leave a 1/2 inch gap between the bottom of the cement board and the PVC membrane.  You are eventually going to bury the bottom few inches in your final cement shower pan, but you wan to leave a gap so any water can run out of the bottom of the cement board.
7. NEVER EVER EVER secure the bottom of the cement board with screws.  I marked where my liner ended and that was the first point from the bottom that I screwed into the cement board.
Here is what my shower and knee wall looked like once I added the cement board:
I did use a drill to cut out the holes for the plumbing fixtures, but the rest of the cuts were all made by scoring and breaking the boards. After the board is hung you need to tape and mud the seams.  DON'T USE DRYWALL SUPPLIES.  You must use special alkali resistant cement board tape and mortar. It is easiest to just use the same mortar you are going to use to set your tiles.  (In my case I am using a mortar specifically for travertine.)  You can see the tape in the upper right corner of this photo: 
I also used mortar to repair the damage to the green board behind my roman tub.  If this were a "regular" tub I would have pulled all the green board out (since it is NOT good to use on a tub) and replace it with cement board.  However, since this is a roman tub and is so deep, and it is so infrequent that water comes in contact with the walls around it, I decided to forgo the time and expense of replacing it.   If you were to install a hand held sprayer or anything else that could possibly lend itself to excessive splashing, you would definitely want to use cement board:

Once the walls were repaired, it was time to finally finish the shower pan. I talked a little bit about how you build a shower pan correctly in my last post.  This was just the final step.  I used this mud calculator to figure out exactly how much deck mud I was going to need for the final pan.   It turns out I ended up needing 40 pounds of portland cement and 200 pounds of sand.

Needless to say I needed help with this step because my mixing pan wasn't large enough to do a batch that large.  I had to mix three smaller batches.  Because the set time was under an hour for portland cement, I had hubs make the mix, while I spent the hour packing and leveling the deck.

I didn't get a lot of pictures of this process, but when all was said and done, this is what my final shower pan looked like:
The dry pack mortar (or "screed")  was so much easier to use than the mortar mix I originally used for the preslope.  At this point you could also pour the curb, but I knew I would probably mess up my soft shower pan if I tried it, so I let the cement cure the recommended 24 hours before tackling the next step. If you ever want to try it yourself and want a little advice, I encourage you to check out the John Bridge tile forums.  They are so much better than any other DIY website for the subject of tiling.  I actually have become a little addicted to reading them.. they are like a tile lover's Pinterest.   I think it may be a sign that I have become obsessed with this project!! 
Next up: Finishing the shower curb and waterproofing.   Budget for this part of the remodel: $103.23Cement board (5 sheets): $57.35Mortar Mix: $21.63Hardibacker Screws: $7.86

Cement board tape: $6.41

All Purpose Sand: $5.64

Portland Cement: $4.34

Renovation to date total: $224.91