As computer graphics imagery (CGI) becomes more advanced it gets harder and harder to tell which images are real and which ones are not. Between movies like the Final Fantasy, Matrix, the Star Wars prequels, and other CGI-heavy laden movies, it seems to me it won't be long before we have virtual actors/actresses. Actually, the mathematical formulas to represent human hair or even water are very complex and as of yet most CGI created images still have a "cartoonish" feel to them and not real-world, but they're getting closer. Who knows what the next 20 years will bring?

Even professional or amateur computer graphics artists can fool the human eye. Take the real vs. fake photo challenge here and pick which images are CGI and which are real:
http://www.alias.com/eng/etc/fakeorfoto/index.html