This is a common error. One of two things will help. First, check your references. When you click on a reference and look at the properties, one property is called "Copy Local". It should be false for DotNetNuke, and most anything else. This usually fixes the problem.
In order to be really safe however, just go into the Solution properties, then Configuration properties, and uncheck all the projects except your PA's. In other words, you don't need to recompile DNN and all it's projects every single time. This helped me a lot, and I never get that error now.
-Todd Davis
http://www.SeaburyDesign.com