I'm ALMOST certain Squarepusher's "Iambic 5 Poetry" is fretless slapping with reverb and a bit of active EQ messing. It's not something I can get out of the fretted basses I've played. I don't own a fretless, but I was slapping on a fretless Jazz in a store one day and felt that if I had more treble, or roundwounds, I would have been able to get this sound. Plus, check out the few slides he does; they sound smooooth.
Listen to it
here.
Please correct me if I'm wrong.