I heard back from Nick, here's what he said:
"As many people have said (including Kim Stanley Robinson, whose Mars Trilogy I am re-reading at the moment), “you can never go back”. Habitable Zone was a fun and important time of my life and I enjoyed most of what was said and done on the site, but I grew out of it. It was taking too much of my time – albeit at a stage in my life when I had a lot of time to fill. These days, I am far more busy, and don’t really have the time for any internet forums.
I will return to the scientific challenges of a frozen Mars at some time – I’ve still got a couple of things to chase-down or work-up, but in many ways the last decade of Mars science has vindicated many of my theories about modern Mars being frozen, but has also revealed a wide range of data about how, at times and places in the past, there does appear to have been significant amounts of liquid water – although in many cases it was more like battery acid than potable water."