I've actually thought about this a lot, to the point that I can't even remember all the names of celebrities I'll miss for their art.
I know Christopher Hitchens, though not quite a celebrity, is currently battling cancer. I was exposed to him in a documentary called Collision, where he and Pastor Douglas Wilson went on tour having debates on whether God/Christianity was good for the world. The man is intelligent, and unlike guys like Richard Dawkins, he's not an asshole. He's currently battling cancer, which is, to me, a sad sad thing.
The day Ridley Scott and Martin Scorcese kick the bucket will be a sad day indeed. It feels like there aren't many directors that pioneer like they did, and Welles and Hitchcock before them. At least, not in the same ratio as back in the day.
I'm going to want to cry when Leonard Nimoy, William Shatner, Patrick Stewart and Mark Hamill kick the bucket (Hell, that Mark Hamill isn't doing The Joker anymore is reason enough to cry).