I just got a whole bunch more too. nanog being subscibed to different mailing lists.
Someone *is* playing with us.
One likely explanation is that a spammer is reading nanog and is showing his/her/its displeasure with the anti-spam tone on this list recently. This sort of revenge is very common among spammers, in my first-hand experience. Block a spammer from your site and shortly thereafter various malicious things begin happening with disturbing frequency. That's one of the big problems with spam. Perhaps most of you remember when, quite correctly IMHO, women's* groups pointed out that rape is not a sexual thing but, rather, a power thing? Well, allow me to point out that, analogously, spam is not a commercial thing... * yes we should all know by now that rape is not, in absolute terms, gender-specific, but nonetheless it was women's groups who were associated with first making this point even if it doesn't strictly apply only to women.