This appears to be related to syslogd, as setting syslogd=NO in rc.conf 
allows the system to boot and be more-or-less fully functional.

This was deduced by noting:

Enabling pf firewall.
Building databases...
Starting syslogd.
Starting pflogd.
Nov 11 13:35:59 pflogd[570]: [priv]: msg PRIV_OPEN_LOG received
Starting named.
Starting named.
Stopped in pid 632.1 (syslogd) at       netbsd:cpu_Debugger+0x5: 
leave
db> step
Stopped in pid 632.1 (syslogd) at       netbsd:cpu_Debugger+0x6:        ret
db>

Basically the pflogd message isn't set to syslog like it was previously, 
and dropping to the debugger shows that syslogd is the current process.