There's not a great deal of documentation regarding the optional 
interfaces, but if I quickly run through what I wan to do can anyone 
tell me if I can do this before I go ahead and waste a day?

One m0n0wall, 5 interfaces, LAN, WAN and OPT1-OPT3.  WAN and OPTs 
plugged into a seperate DSL router with two IP's on a /30.  I want to 
run IPSEC endpoints on each of those interfaces and route that traffic 
to the LAN.  Will that work, or am I using the OPT interfaces for 
something they weren't intended for?  Will I need to create rules for 
all this traffic or will they, like IPSEC to the WAN interface, "just work"?

I only really want to do NAT for 'net access on the WAN interface, the 
other interfaces will be purely for IPSEC.

Cheers,

Kev