Good post bart, nice to see that windows has some decent commands relating to standard internet protocols. In the longer term, you could have a permanent method of spotting rogue dhcp servers. I found a rule for snort:

#
# DHCP Servers
#
alert udp !$DHCP_SERVERS 67 -> 255.255.255.255 any (msg: "possible rogue DHCP Server"; sid:1000001;)

I know you use nagios, I dunno if theres a plugin for that yet.