Well I went to my gamestop today to see what some of the old gamecube games I had were worth (not much -_-). After telling me the prices, he told me that if I was gonna trade them in, I should do it soon because they are gonna stop buying the used gamecube games. He couldnt give an exact date on when it was gonna happen, but I think it will soon (possibly Early/mid 2009).
Note: Im a Nintendo Fanman, I did not ask about xbox or PS2, I could careless about those systems.