I had this problem with my MBP 2.0ghz, it would also spit out all DVD/CD's unless they were new. I took it in to apple care, and they asked me to put one of my DVD's so they could see it for themselves. It did not accept my DVD, nor did it that of the genius bar tech. They ordered me a new optical drive.
I would just take it in to the genius bar Im sure they'll replace it for you.