I would assume it's similar to Oblivion, the remains go when the cell respawns, if you have stayed away for 72 hours or whatever. Damn shame, making a 'Dragon's Graveyard' of your very own would be pretty cool.
that is until the game becomes so cluttered with dragon skeletons, the game starts breaking apart and becomes completely unplayable... kind of like fallout 3 after only a couple hours of gameplay.
even more so when using energy weapons in fallout 3... god, i hope the dragon remains will vanish after a couple days.
dragons are infinite... its going to be a REAL damn pain if they stay forever... for the reason i just mentioned, and because it'd just be irritating having to navigate through a maze of bones while trying to find a dungeon to explore.