Thanks for the clarification - I forgot the PPTP clients actually use the /32 subnet when they are connected.
However, from the server point of view it does not even know about this - all it knows about is it's own subnet mask.
Thus, if the PPTP client is considered by the server as being in the same subnet then it will not attempt to send the traffic to it's default gateway.
Maybe I'm wrong - but this is just my understanding of IP and subnets.
To check whether the ICMP REQUEST packets are actually getting to your server but the ICMP REPLY packets are not routed back properly, I suggest you run ethereal/wireshark on the server to capture this. If there are no PING packets getting to your server in the first place then there is routing/IP problems elsewhere.