As far as I remember Cemplank before being purchased by Hardi was pretty much the same product but the trim was slightly different (don't remember how now). The pricing if I remember was slightly less but after being purchased by Hardi I thought the name went away so in all honesty I have no clue what the difference is today.

I know the OP is from 8 years ago, but wanted to add some information we received from our contractor. For those who may be searching for answers. Cemplank warranty is 25 years vs Hardie warranty of 30 years. Our contractor did offer us the option of Cemplank because it is cheaper (would have saved us approximately $1 per square foot), but it's cheaper for a reason. True Hardie siding has certain moisture/flexing standards to accommodate harsh cold and high heat/humidity. Cemplank is not meant to handle harsh cold or high heat/humidity and does not have the flex capability of true Hardie. He told us that most new construction here in Middle Tennessee is actually using Cemplank for the siding. Home builders are stating that siding is Hardie, which is technically true since Hardie acquired Cemplank, but home buyers who read the fine print will find that they are not getting the true Hardie with the 30 year warranty.

While we do typically have mild winters here we can get some pretty intense heat and humidity so we opted for the traditional Hardie board. He was able to show us pictures of fairly new homes that had been built with Cemplank that he had been called out to do repairs on. Fairly new being less than 5 years old. Just throwing this out there for what it's worth.