I wonder about Dean's immortality too, and I have a notoriously shitty memory. Dang.

Is it because with the Mark, if he dies he'll just return as a demon anyway? Or is it because Cain lived (peaceably) with his bees and the Mark, until the Winchesters and Crowley stumbled into his bucolic life, so we're assuming the Mark grants immortality against mundane disease and aging?

I don't knooooow. *whines*