I know this is possible because one of my older solution files has a mix of some projects under source control and some not.
However I can't seem to reproduce that configuration with a newer solution. Every time I add a project to the solution it acquires a yellow cross and under Pending Checkins the project is listed as a Newly added file.
This is with the Perforce plugin for VS2008.
I have tried adding a brand new project, vs an existing project. I have tried p4 obliterate. I have tried disconnecting and reconnecting source control. Binding and unbinding.
After all this, every time I try and add the project it gets a pesky little yellow cross. I can't seem to tell Perforce and VS that, while I do want source control on the majority of the solution, I don't want this one project file under source control.