Admittedly, this is of no use to the average Internet user, but there's a Firefox addon called Certificate Patrol. It alerts you when an SSL certificate changes. It shows you the old cert information along side the new cert information. It tells you if the old cert was due to expire, and also if the signing authority has changed.
Certificate Patrol looks amazing - amazing enough to switch back from Chrome to Firefox in fact (now that Firefox is a bit snappier). Pity it's not available for Firefox 4!
There's also Perspectives.