Temperatures are hovering right at 32degrees in Denver right now and will be for about 24hrs. We turned on the sprinklers this week, of course not anticipating this. The 1in copper supply line is exposed (not insulated0 for about 2feet from where it leaves the basement and goes outside to the sprinklers. We have not run the sprinklers yet; but the main water line to them has been turned on. Do I need put some insulation on it (foam wrap stuff) and/or turn off the main water line to it?

I'm not an expert with sprinklers but this appears to me to be a professional recommendation:
Prevent broken sprinkler pipes in case of a sudden freeze - YouTube (http://www.youtube.com/watch?v=WW4VYWamwwo)

The below reference is also good and will lead you to possibly (your decision not mine) but it may lead you to do nothing if you trust the weather forecast.
An excerpt from the link below:

"This threshold is based upon research conducted by the

Building Research Council at the University of Illinois. Field
tests of residential water systems subjected to winter tempera
tures demonstrated that, for un-insulated pipes installed in an

unconditioned attic, the onset of freezing occurred when the

outside temperature fell to 20 degrees F or below..."

I have heard this from farmers worried about waterlines to their troughs (water needing to get down to 20 deg F to freeze and burst a water line) for a long time and I'm not a spring chicken but have no scientific proof to offer. At minimum, insulating the components subject to outside air (see video) may be advisable. In spite of everything I have said so far, and this being an expensive system, I would probably follow the advice in the video as it appears to be not too difficult to do.
http://www.iccsafe.org/cs/PMG/Documents/DIS-FreezeBurstPipe.pdf