In PIX 6.3 and later, support for PPTP through the PIX has been added.
Before version 6.3, a PIX needed to be configured to allow the GRE tunnel back from a PPTP server.
As of version 6.3, the firewall merely needs:
fixup protocol pptp 1723
Prior to 6.3, the following was needed:
static (inside,outside) outside_ip inside_ip netmask 255.255.255.255
access-list pptp-acl permit gre host remote_pptp_server_ip host outside_ip
access-group pptp-acl in interface outside
The Cisco document: http://www.cisco.com/warp/public/110/pix_pptp.html