Did you change the frequency of notification e-mails? Yesterday cereal voted on a post, and I got an email today. I've noticed this last week too, yet remember getting them almost instantly before that.
I have not been receiving notifications for about a week, but today they started coming through again, although for posts that are days old. Guessing there is a Gremlin in the system.
Same for me, I just received a notification sent 4 days ago.
There was a severe problem with our email server that had been going on for about a week. Unfortunately, no one brought the issue to my attention, so I had just found out about it yesterday. Problem resolved now :)