Another update. Vincent Manelschots reported the same problem in Google Groups and indicated he found a workaround. I contacted him about it and he reported that.
When you compile the probject in pb.net it crashes with the error. When you go to the output directory and locate the projectname.csproj file en open this in your favorite editor. Then locate the line
<Reference Include="presentationcore">
<HintPath>(somepath)</HintPath>
</Reference>
If you delete the hintpath element then it will compile. Unfortunately it will override the file on serveral occasions (when you change any reference for instance) and the build will fail again.
That works as long as you aren't referencing any 4.5 classes in your workspace. I still get the error once you include a 4.5 class, and am still following up with SAP tech support on that.