My theory on the mystery and secrecy of Albert Finney, which to me if he was playing a superior or something he would be labelled or known as (for example) - Albert Finney as Dalton, Head of Foreign Security (something like that).
I think that he will be playing the new M.
With what we heard today about 'Skyfall' being emotionally linked to something in the film, and the synopsis about M's past, I think that during the film either Judi Dench's M is killed or taken off duty, rocking MI6 and maybe leading to even more of a character development tool for Bond, but also maybe scaring Eve (Harris) off duty to becoming a secretary, aka Miss Moneypenny.
I think Judi Dench's M is now in her final film in a story that could shock as well as deliver the suspense and twists like 'Casino Royale'. Albert Finney is drafted in as the new M for Daniel Craig, be it for just this film until the MOD appoint a new Head, or simply like Bernard Lee did, appear for a few minutes of the films and then be done.
That's my idea anyway! Can't wait to find out.