This has been an ongoing problem with some of the Apple SuperDrives, with various ideas on the problem but no actual solution except optical drive replacement. Hundred of people over at Apple Discussions have had similar problems, and some of the solution ideas have included compressed air or even removal and lense cleaning of the Superdrive. I too have had this problem on an out-of-warranty pre-unibody MBP and none of the fixes I've tried have worked, yet.
http://discussions.apple.com/thread.jspa?threadID=1918925