Hi,
Actually you need to make sure that the device that is doing PAT (a FW) that your PPTP users are sitting behind, is PPTP aware PAT e,g, a Cisco router having 12.1(2)T or later support PPTP PassThru feature.
You can try putting a static translation entry for a particular host on the FW (doing PAT) or connect via dial-up account to double check thats what is happening because of PAT.
http://www.cisco.com/warp/public/110/pix_pptp.html
PIX debugs should also help here.
Thanks,
Afaq