It's the type of shim JoeJack951 linked to. I got it from the fitter at a bike shop in my neighborhood this past weekend. He suggested that it would help distribute the pressure evenly across the entire width, instead of only where the stem connects to the bars. I had the same problem before I put the shim in place, when I was using a 26 mm stem, but it seemed less pronounced. I'm not really sure.

The 31.8 mm stem is very adjustable, and I haven't been able to find the same thing in 26 mm yet, which is why I'm using the setup I've got right now. I was getting sore shoulders after long ( three hour ) rides, and moving the bars around is one of the things we did to fix that.