I have an updated version in pkgsrc-wip that I have been testing.

But I don't want to commit to official pkgsrc yet since I get database 
corruption on multiple spamd servers at least once per month (for the past 
six months or so).

It would be a good idea to work with the FreeBSD spamd maintainers for the 
spamd portability. (FreeBSD's version has an optional build for ipfw.)

http://freebsdspamd.berlios.de/

As for your problem, it assumes your 3.1 system has the reentrant bits.
See patches/patch-aa.

D'Arcy: can you look at this? (see pkgsrc/mail/spamd/patches/patch-aa)