Even Movable Type has problems with spam, and they've been talking about ways to combat it.
Personally I'd have thought the easiest way would be to have a page which lists all the recently posted comments, in chronological order... that way the blog owner would be able to see comments made on months-old entries.
Actually, it'd be nice if LiveJournal had something like that (but then it'd be nice if LiveJournal had Trackback and categories...).