Yes mainstream games all seem to be rehashes these days, but so what? People make what sells. Actually, it does bother me a bit, but i reckon this will change in time.
What we have now, and are going to have in the future is more like the film industry- you have your by the numbers blockbusters, and you have your indy films.
With any luck, the better ideas and artists/players in indy games will end up making their way onto the "big screen".
Things like Xbox live show promise. Also as next gen consoles get older and 3rd party libraries become more available, we'll hopefully see more adventurous games.
It's cool that it's getting to the point where computers are so powerful that even if you get a second rate cheapo graphics engine or whatever and it's only giving you 10% of what's possible, you can still make a great game. Bit like how you could make an enjoyable movie on a cheap camera if you wanted to.