I did some test with openvpn and got it working with using windows 7 as client and nwsp as server using multiclient conf files.
Although I can scan IP's in the vpn network using ip adresses in for example Nmap-Zmap gui and then see all ports and service that are available on that ip and I can also open the samba shares of the nwsp2 using the vpn ip, windows 7 doesn't show the VPN network in network neighbourhood. Sometime it list the twonky server as being available from the VPN IP. Not much difference so fat with pptpd.
This is not malfunction of vpn but more of the way windows detects and scan networks. Maybe Android can do a better job.
Anyhow openvpn works ok as well
ps iptables is not required for openvpn, iptables is more a firewall application. Using the route command is also very helpful