Joe,
I watched one of the support engineers where I work cleaning out a Compaq laptop today that was running "hot, loud and slow". I saw the amount of fluff that came out of it - lumps of dust the size of my thumbs!
Without dismantling it I don't know if compressed air will help much - I'd have thought that a powerful vacuum might actually shift more dust out of the case than simply wedging it in further by blowing!