Thanks to some people on the m0n0wall mailing list (Edward Mzj), who worked hard to add new features, it is now possible fot m0n0 to use an interface as a span port (also called 'mirror port'). A span port is usually used for packet sniffing and, more importantly, for Intrusion detection system usage. With a span port, all the traffic going through interfaces is 'copied' to the span interface so it can be sniffed by another computer. I'll finally be able to run an IDS at home without buying an (overkill) smart or manageable switch! Thanks guys!