"VPN endpoint (Server) functionality is one that is missing in WL-500g firmware and i think it is quite usefull not only the one that has already bought a WL-500g device, but also to new buyers.
I wonder when (if ever) ASus will include VPN Server functions in a new wl-500g firmware release."
I've sent this message to asus support team, but i didn't get any reply yet.

Is it too difficult to add VPN server to wl-500g firmware?
I remember that it's available to SL500 and SL1000 firewall/vpn devices by ASUS.