I tend to agree with Skypilot, that the hypothermia thing is unlikely to set in as long as you are continuing to run (ie. not stopping while out in the cold). 

I have had a couple pretty miserable wet, cold long runs that I've written about here, and I think the issue is not so much the hypothermia as much as the rate of energy consumption.  I don't have any clinical documentation, but it seems logical to me that in cold, wet weather, when your body is trying to stay warm, you may use more energy.  I did a 21 mile run in mild conditions with good results, and then two weeks later did a 21 mile run in cold and wet conditions, and bonked horrifically.   It was a freak show....one of my worst running experiences ever.  I learned that I needed to consume a lot more carbs and electrolytes in the cold. 

Unfortunately I think the issue with rain gear is exactly as you say, GinnyinPA.  A rain jacket starts to feel like a Hefty garbage bag in about .02 miles.  I think the best you can do is a tech shirt / shorts/pants that will not absorb and hold so much water so you're carrying the extra weight.  If you're wet, you're wet.

I do love running in the rain.  But as I say...that's at 60+ F.