zandoval 

One thing I learned in the military that applies well to minimalist campers...
It does not rain in the military. It rains on the military. And when it rains on the military, the military gets wet.

To be more direct, if its hard rain or prolonged rain you are going to get wet. You most likely already know this. I use a two man light weight tent call the Segaro that does not cost much. That way when I get it all messed up its not a big loss. I am a fat guy. So a two man allows me to get me and my gear inside the tent.

The Wenzel Starlight and the TexSport Willow Bend both come in at about 3.5 pounds and are cheap. The WIllow Bend is not much more than a rain-fly and ground cloth with mosquito doors. Both will not keep out hard rain, but a thin sheet of plastic over the top of them will. I am mentioning these cheap tents because these are the ones I have seen survive Boy Scout camp outs. That's not to say they looked very good a year or two out.

As for the original post I am sure there is a water proof sleeping bag that works but I have not seen one and I am pretty sure I could not afford it if it did exist.