Microsoft Knowledge Base Article 303684 contains the following summary:

This article describes how to configure IP address reservations for Routing and Remote Access Service (RRAS) clients.

When you configure a DHCP reservation, you need a MAC address, but remote access clients (such as PPTP) do not have a MAC address. Because of this, traditional DHCP reservations do not work. You can manually assign an IP address that is based upon a domain account instead of a MAC address, although the user has to be a member of a Native mode domain.