wiredup,
The Water Authority recommends 2 INCHES? (Did you mean feet).
Well here's my input on depth based on my research before I installed my irrigation system.
1. I believe that plumbing codes typically require any mainline pipe (pipe leading to the control valves) not made of metal to be buried to a depth of 18" (as measured from the top of the pipe to the soil level).
2. Deeper is safer (more protected from freezes and potential vehicle traffic).
3. 10" to 12" were the most common suggested depths for latteral lines in irrigation books and www.irrigationtutorials.com (here's his page on pipe instilation: http://www.irrigationtutorials.com/instal14.htm )
I could definitely see where shallow installations fail within 2 years not so much from freezes but because people forget the underground pipes, and then run over them with a vehicle. A simple example I can think of is you purchase 12 bags of mulch for that landscaped area and you load them up in your SUV. When you get home, you realize that if you park at the curb, you are going to have to make 12 trips to haul those bags from the SUV to the landscape area. You decide that it would be a lot easier to just drive through the lawn, drop the 12 bags on the ground, and drive back to the curb. You forget about those pipes, and a shallow install results in broken pipe.
I've never hear of freezing temperatures affecting PVC, assuming there isn't ANY water in it. 32 degrees is just a magic number for water, not PVC.