Yup, I agree with Glenn that it would work.  The idea is that the warmth of the earth is fighting the cold of the air and the temperature gradient in the ground displays the result.  So if you can put a layer of insulation in the ground, it will allow the heat from the earth to rise up closer to the surface.  I'm believe the width of the insulation is possibly more important than the R value.  
 
 My wild guess would be that as long as the R value is 4 or better, for every 6" of insulation width (from the centerline of the pipe) you could bury it 6" shallower.  So if you had a 2' wide piece of styrofoam you could bury your pipe 1' above the frost line.  
 
 Please note that I said this was my "wild guess" and it's been influenced by a bit of Amaretto this evening 
