Yeah, I did the same thing right before the 360 came out - since I heard it was backwards compatible, I bought X-men Legends II, Ultimate Spider-man, and a few others for the X-box instead of Gamecube, figuring, "I might as well get some games for this new system" and then they go and make SOME games compatible and not others.
It would be nice if at least NEW games, especially ones that will obviously sell well, would be backwards compatible from the get-go, kind of like how they did with Lego Star Wars II.
Here's hoping that backwards compatibility list gets updated soon!