Why does VS2010 remove my references?

In a small console application I'm writing to test something I need to add a reference to one of the DLLs built from a build of another solution. The Add References dialog works perfectly; after I browse to the DLL on disk and add it I see it with a green tick and it appears in the list of references in Solution Explorer. All fine so far, but when I close the Add References dialog, the reference just added simply disappears from th开发者_如何学JAVAe list of references in Solution Explorer.

What on earth is going on? Why is this happening and how can I fix it? Any ideas?

Referencing a non-Silverlight DLL in a Silverlight will appear to work until the Add References dialog is closed, when the references simply disappears from the project's list of references. No warning or error message is displayed.