I actually am doing a workshop at an Art Fest in Indianapolis in June on how I incorporate my ATC's into greeting cards! I have loved using them this way for awhile now. I generally add a note inside the front of the card, on the bottom, explaining that the card attached on front is a collectible work of art- but some keep them- some not, and others ask me if I want them back!!
Here are some examples of them that I have made-