Problem. It is not possible to access NATed services using the public (WAN) IP address from within LAN (or an optional network). Example: you've got a server in your LAN behind m0n0wall and added a NAT/filter rule to allow external access to its HTTP port. While you can access it just fine from the Internet, you cannot access http://your-external-ip/ from within your LAN.
Reason. This is due to a limitation in ipfilter/ipnat (which are used in m0n0wall). Read the ipfilter FAQ for details. m0n0wall does not (and probably will not) include a "bounce" utility.
Solution. If you use m0n0wall's built-in DNS forwarder for your LAN clients, you can add one or more overrides so that they will get the internal (LAN) IP address of your server instead of the external one, while external clients still get the real/public IP address.
This will only work if you use m0n0wall as the primary DNS server on your LAN hosts. If you use another DNS server, you need to use its functionality to resolve that host to the appropriate private IP. See your DNS server documentation for more information.