[BSDfan666]

You really need to stop giving FreeBSD advice in the OpenBSD sections DutchDaemon..

To specify interfaces dhcpd(8) should listen on, add change dhcpd_flags="" to something like dhcpd_flags="xl1 xl2" in /etc/rc.conf.local.

See: http://www.openbsd.org/faq/faq6.html#DHCPserver

Perhaps something like this will work?

shared-network WIRED {
	option domain-name-servers 10.100.0.1;
	subnet 10.100.0.0 netmask 255.255.255.0 {
		option routers 10.100.0.1;
		range 10.100.0.2 110.100.0.254;
	}
}

shared-network WIRELESS {
	option domain-name-servers 172.16.0.1;
	subnet 172.16.0.0 netmask 255.255.255.0 {
		option routers 172.16.0.1;
		range 172.16.0.2 172.16.0.254;
	}
}

This configuration assumes xl1 and xl2 are configured to have the addresses 10.100.0.1 and 172.16.0.1 respectively, and that a DNS server.. like named(8).. is listening on both interfaces.

Note; I'm not aware of any wireless 3Com/xl(4) devices.. so that was probably a typo.