This happens when executing inside the Visual Studio 2010 process, but not when executing inside the default MSTest process (use the VS SDK's "VS IDE" HostType for repro).
When an assembly is loaded in a new app domain inside Visual Studio 2010, the assembly file handle remains locked after the app domain is unloaded. This appears to happen when the assembly is loaded into the 'Load' context, but not when loaded into the 'LoadFrom' context.
This issue could impact any add-in or package that needs to execute or examine assemblies in an isolated app domain. The most likely symptom is the target project will fail to build until Visual Studio is restarted.