After upgrading to 1.2.1-RC3 on Monday, and letting it run a few days, the problem appears to be gone.
USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND
root 386 0.0 0.2 5696 1960 con- S Mon12PM 0:05.94 /usr/sbin/tcpdump -s 256 -v -l -n -e -ttt
I'm still not sure what may have caused the symptoms I saw in my original post. Something sure made tcpdump chew through a lot of CPU time and memory in less than a week, but as long as it doesn't happen again it was probably a fluke or a side effect of the testing I had done.