Atmospheric "Saturation"

There was a debate recently between the RealClimate folks:
and Lubos Motl, on his "Reference Frame" blog:
regarding whether or not the marginal increase in global warming should lessen as more and more greenhouse gasses are added to the atmosphere. I made a few calculations on the subject for myself, eventually coming to agree with RealClimate that it need not. And yes, it pains me to say so. A slightly mathy write-up, in pdf format with spiffy imbedded links, is here:
Check it out if you are interested. It's intended to be self-contained for the most part.