Akismet doesn't trap them nearly as well as comment spam, and sometimes it's even hard to tell manually whether they're spam, since the spam-blogs have gotten good at stealing plausible-sounding content. Most of the time it's only by the 3rd identical pingback from a different domain that I'm sure.
Are pingbacks worthwhile at all? I don't get very many, but at least half of them seem to be spam.
Current Music: Giana Factory -- Bloody Game ♬