Have you ever created a class library project, started to write some unit tests and realize you did not use the VS2005 UnitTest Project Template to create your project.
I have done this countless times, left over habits from NUnit days I guess.
So, using notepad, I popped open a csproj file the I created using the correct VS2005 Project Template for unit testing. I found a key called ProjectTypeGuids. Using notepad again, I opened my class library project and did not find a key called ProjectTypeGuids. So, I put the following key in my project, and what do you know it worked!!!!
I think the guids are different for VB.NET and C# project templates. So if you are using VB.NET, you may have to lookup the appropriate guids.