Derik Whittaker has posted a blog post that I found recently via stackoverflow.com that demonstrates a very unknown feature in the Add Existing Dialog in visual studio. Adding as a link.
This is… super useful.
I think they took this out of VS2008, or unless I'm not seeing it or using something else.
Nope, it is still there, the add button is in a slightly different place.
Just Add Existing File, and click on the arrow ON the add button.