William Hartnell was dead even before The Five Doctors, in 1983. The role of the First Doctor was played by Richard Hurndall, and he's[/ dead.Well apparently Hartnell wouldn't return Moffat's calls...
Of course, there are those who don't rate Hurndall's performance. I've seen it argued here and elsewhere that there's no point in having other actors play Doctors 1-3, that those roles are intrinsically linked to the original actors. I'm not necessarily in that camp myself (I'd like to see Sean Pertwee play his father's role) but I can see its merits. I suppose, for the sake of argument, while you're at it, why not cast Robert Carlyle or Sean Bean as Nine once Eccleston refused to return ?