I like the idea of seeing people's photos from the past that fit the current project, but I also like the idea of seeing the photos taken for the project.
I think it would be a good idea to have people post previous existing photos in seperate threads or have two stickied project threads, one for existing photos and one for new.
As an example I posted an "Old Colors" thread for some old shots but will put new ones in the project thread.

I agree 100%! I have posted a few old photos in the current projects threads, but I always make sure that I label the photos as old.

I have always thought of the projects forum as an assingment to go out and make new images that fit that project.