While IPv6 may not appear to be critical to the server/client communications relationship, SBS 2008 does require IPv6 for some of the server based components to work correctly.
- Official SBS Blog: Issues After Disabling IPv6 on Your NIC on SBS 2008
One of those unintended consequences may be the inability to configure the PPTP VPN setup in SBS after disabling IPv6 on the server.
The error will state that RRAS is not ready to be configured.
The above blog link contains the proper methodology for disabling IPv6 and part of that process is how to allow PPTP to be configured by the wizard.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book