It sounds like you haven't gotten much use out of your siphon since installing. I know the dry weather has been rough on a lot of people. We were real dry last year, but this year has been a complete 180 so far. You can almost stand outside and watch the corn grow this year.
I found some good information on common PVC failure referring to freezing water/solution inside the pipe. Basically in a nut shell, radial expansion is limited, or constrained, by the pipe walls which forces the ice to expand longitudinally. Linear ice expansion will cause a build-up of pressure between inlet/outlet valves, and it is this pressure which will expand the pipe, causing the wall thickness to thin beyond spec. and ultimately to failure.
As long as there is a properly functioning vent (vent itself hasn't developed an ice/debris plug) radial expansion would be limited down to a non-issue. Those incorporating butterfly valves on their outlets and ball valves on their vents may want to be conscience of their position during freezing weather.
And ultimately, if you want to safeguard yourself from any possible radial expansion, splurge a little extra on the inlet pipe and go with SCH 80.
