I'll be providing details soon, I had some things happen that require my attention for a bit. The preliminary test went very well, though I used only 2" bottles of frozen water (video can be seen here
http://www.hauntforum.com/showthread.php?t=2761. The fogger is 700 Watt.
I plan on using the 3 1/2" water bottles for the real thing, which if I remember my geometry will provide a volume of 4 1/2" squared between the 4 1/4" pipe and the 3 1/2" frozen water bottles, which is slightly more than a 2" pipe would provide. However, if you can imagine a 3 1/2" frozen cylinder centered in a 4 1/4" pipe, it seems like that should provide a lot of cooling, especially down the length of a 12' pipe.
One difference from other PVC pipe chillers I have seen is instead of adding a T to the inlet for venting, I used 2" PVC for the inlet, and left an air gap around the outlet of the fogger and the 2" inlet pipe. barely any fog leaked out. I also found the large elbow at the end was not neccesary since I was using frozen plastic bottles there was no need to retain ice / water in the pipe.
The result is I will only need to purchase a 4" to 2" PVC reducer (I also found a "3 to 2" reducer works too, it fits snugley inside the pipe), and a short peice of 2" PVC (mine was about 2'). I will work up a complete how-to soon.