From: "M0n0wall" <m0n0wall at toyne dot cix dot co dot uk>

> I'm not sure if this is related, but I have just been playing with two 
> wifi connected PCs and noticed that they cannot talk to each other. I can 
> quite happily ping m0n0 from either, but not each other. The wifi router 
> is effectively being used to pass traffic directly to m0n0.

On this you probably have access point isolation enabled on your AP.  It 
"supposedly" adds security by not letting clients see each other, but in 
practice it just makes things harder.

                        Lee