I just realized why I sometimes miss a few comment spams: when I receive the email notification, Apple Mail flags and files some of them as junk, which is only logical considering their content. So I have to watch my weblog Junk mailbox or find a way to prevent Mail to touch those.
Now I still don't understand why spammers bother to comment on 6-month old posts that get absolutely zero traffic. Zero traffic, zero PageRank or value in search engines thanks to MT redirection, zip, nothing, nada, useless. Hello !?! Gratuitous nuisances.