As abruptly as it started, the flood of inbound spam seems to have stopped. The only thing that I actually did that might be related was to switch my secondary MX from sendmail to postfix and put some spam filtering stuff on there. Apparently a lot of the spam that I was getting was rejected by my primary, but accepted by my secondary and then forwarded back to the primary with the added credibility of coming from a trusted host. Or something like that.
Either that, or the sobig.f virus hit its timeout, and quit sending. Or perhaps mrtg broke and isn’t reporting stats right.