I just set up a pptp server on Debian Linux yesterday by following this guide: http://poptop.sourceforge.net/dox/debian-howto.phtml When I tried to connect to the server using both XP and iOS, the client successfully connects to the PPTP server, however, I can't seem to access the Internet thru the pptp connection. When I tried to capture traffic using tcpdump on just the pptp interface itself, I can see only outgoing traffic and no incoming traffic at all. (Please see image)
more than likely, your internet gateway (router?) does not have a route back to your PPTP gateway. So when a packet comes from x.x.x.x (VPN ip-range) the router throws it to the WAN interface instead of back to the PPTP server.