The system crashed again with a kernel page fault at the same PC. Despite having set debug.ddb.textdump.pending=1 it didn't reboot automatically so Ityped in the commands in kdb.enter.panic one per line and then on reboot savecore reported it was writing textdump.tar.
This system has a VIA C3 CPU and is running
dmesg
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FreeBSD 8.1-RELEASE-p2 #1: Mon Apr 18 23:21:20 EDT 2011
sullrich@FreeBSD_8.0_pfSense_2.0-snaps.pfsense.org:/usr/obj.pfSense/usr/pfSensesrc/src/sys/pfSense.8 i386
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: VIA Samuel 2 (797.74-MHz 686-class CPU)
Origin = "CentaurHauls" Id = 0x673 Family = 6 Model = 7 Stepping = 3
Features=0x803035 <fpu,de,tsc,msr,mtrr,pge,mmx>real memory = 268435456 (256 MB)</fpu,de,tsc,msr,mtrr,pge,mmx>
I'll switch to a debug kernel as suggested.