1811 router lets me get as far as "monitor session 1 source interface fastethernet" but when I finish it with the interface number "fastethernet 0"
it gives me a "Invalid input detected at marker |^| pointing at the F in fastethernet.
The Cisco 1800 Series Integrated Services Routers (Fixed) Software Configuration Guide claims SPAN is possible. Page 5-9
I believe this is because the Fa0 and Fa1 ports are technically router ports (that can never be used for switching) on the 1811, while the Fa2 through Fa9 ports are Layer 2 switch ports (which you can set up span sessions for).
SPAN is a switch technology, not a router technology (as far as I know). It's more a router with layer 2 switching ability, not a switch with routing capability (like the 6500 series Cats).
Just curious, but why do you want to SPAN a WAN port?