It occurred to me that David Young wrote in gmane.os.netbsd.current:
> dhclient sets the interface to promiscuous mode.  That usually entails
> fiddling with the Rx filter.  Maybe that shakes the receiver out of
> its slumber?

I tried that. Restarting dhclient closes the last remaining open BPF, so
I tried that too. Nothing. 

And, the hardware itself is *not* dead. I can access my modem. When I run
tcpdump, I see traffic for my public IP coming by - nobody seems to be
responding though.