Appropriation has been a sensitive topic in art for years. To me appropriation is okay when your work can stand on its own. It is not if the artwork you're using makes your work. Art is all about expressing yourself and how you interpret the world. However, in today's culture people are coping other artist's artwork and changing them slightly by calling it their own. I don't think I would take this problem to an extreme as placing a law on it but it should be a known  faux pas or a unwritten law to respect fellow artists. Appropriation is appropriate when doing practices or lessons on a certain subject. Altering other artist's work for experimenting is definitely okay and should be done to help develop your success as an artist. I do not think it's appropriate when you take a known artwork and display it to the public as  your own. Appropriation can in some way count as its own form of art. For collaging it is definitely necessary to have influences or other photos and materials from other artists, however your collage should look nothing like the original work and it should have your own personal touch to it. If every artist copy off one another there would be no originality left in art. There should be a happy medium with appropriation. I no i am personally influenced by other painters and it helps me develop further as a painter. But i would be personally hurt if someone used my ideas and called them their own. I am on the fence about appropriation. I think if it is done with respect and class then it could possibly work. 

http://www.artofcolour.com/in-depth/mona-lisa/duchamp-mona-lisa.JPG

http://blog.seattlepi.nwsource.com/art/library/david_in_america%5B1%5D.JPG