Since comment spam has been increasingly occurring on WordPress blogs lately and I don't want to have to react only after it lands in the spam folder, I've proactively installed Spam-Karma. It's a pretty powerful tool where, fortunately, you can disable a lot of options. I hope this will prevent what will certainly be an onslaught on my comment function at some point.
Of course, such a tool always has potential negative side effects. So if you can't get a comment through, there's still the regular feedback form which sends a completely normal - and unfiltered - email to me. As long as it makes it through my mail spam filter, I'll know what's going on (with 300-400 spams a day just at home, I can't guarantee that I'll notice an email that was mistakenly flagged as spam - though apparently not much gets lost that way, statistical spam filters do their job).
It's kind of strange how we have to artificially neuter our communication tools just because people tend to exploit anything that can be exploited eventually...
Update: after it ate a trackback from Schockwellenreiter, I've disabled it for now. The main problem was that the trackback was eaten with an error message that supposedly was fixed in exactly the version I'm using.