After a reboot on one server (with backported Samba for Sarge IIRC) Syslog started logging "dhcpd" messages using the facility "nss_wins". Restarting dhcpd seems to have sorted the immediate problem (it emailing me DHCP output).

Some discussion of a similar issue here;

http://www.unixadmintalk.com/f61/postfix-syslog-problem-231090/

I'm sure this is probably a long fixed bug upstream (even Etch), but does anyone understand what is going wrong here? The syslog service seems to be the classic C programmers security nightmare, and this doesn't add to my confidence in it.