As an adendum to this see the OpenVPN docs page:
http://openvpn.net/index.php/documentation/install.html?start=1Particularly the Notes -- Setting TAP-Win32 address/subnet automatically via DHCP
"The problem with getting addresses for VPN clients via DHCP is that you only want to get the IP address and subnet mask, not the gateway. Therefore in a bridge situation, a DHCP server must be able to differentiate between local clients and remote VPN clients."