For those of you who have never seen Mitch Hedberg, think Steven Wright with a drug habit. He was probably one of the funniest comics I'd ever heard. Apparently he was found dead as a result of heart failure. He had a congenital defect which he had expressed some concern about to friends and family.
Comedy Central has a writeup on his death here.
If you get a chance, I definitely recommend that you download some of his stuff and listen, or better yet, go out and buy some of his stuff, you cheap bastard.