You are right, the water bottle or water bladder idea is certainly not new. I've been using mine for years and am very happy with same.
The picture above shows how round, thick and soft my "pillow" looks like.
Thickness shown even better.
My "pillow" is made out of 4 different components (ALL multiple use): my water bladder (2.0 L Platypus), a Buff (as pillow case), the blue item inside is my Cocoon vest and in between the bladder and the vest is a MYOG back-protector (not seen). This last item is part of my TFD backpack and has 4 different uses), it can be separated from same and is very useful in my pillow system to avoid the vest from "sliding down".
The TFD "back protector" in use with my MYOG backpack.
The vest is my only spare part of the clothes I take into the mountains and if it really becomes cold at night, I'll wear it. In that case, I'll put my shoes under the Platypus, although that certainly isn't as comfortable. I must say it doesn't happen very often.
I always use the Platypus almost completely filled with clean water. Having the Platypus nearly full, gives it enough thickness and firmness; the clean water brings an added advantage: I don't have to filter any water in the morning before I can have breakfast (and morning coffee).
Billy Ray: >But if you're going to use a water bottle, why not one of the various bladders? softer and maybe even lighter.<
It certainly will be lighter; my Platypus is only 36 grams (1.27 oz).