I spoke too soon. Not only did this continue to be an issue on nanobsd/net5501, but I changed hardware and software version and it continues to be a problem.
I'm now running 2.0-BETA4 (i386)
built on Thu Oct 14 01:16:12 EDT 2010
FreeBSD 8.1-RELEASE-p1
on a SM X7SPA-H (Atom D510, 4GB) and seeing the exact same symptoms: mbuf usage increases steadily until uptime reaches approx 7 days, then total hard lockup.