From: "JT" <j dot tarot at tecsas dot fr>
> I've no control on cisco (1700) routeurs which are managed by the ISP.

This could be a problem.  You will need the cooperation of the ISP.

> ATM, I've made the base setup so, from the m0n0wall interface, I can
> ping the ciscos. But no chance from the LANs.
> I've tried to add a rule that allow all trafic from "SDSL nets" on
> opt1 interfaces to any... still unsuccessful.

You need static routes to the Cisco routers for the other LAN.  However, the 
Cisco routers also need those static routes.  It sounds like the Cisco 
routers don't know about any of the 192.X.X.X addresses, so the packets stop 
there.  Try a traceroute, and I bet you fail at the Cisco.

                        Lee