Just now got this working myself after much head scratching. I turned off Back To My Mac (BTMM) because I noticed, on this helpful page of port numbers:
http://support.apple.com/kb/TS1629?viewlocale=en_US that port 4500 is shared amongst VPN and BTMM services. Finally I can get back to VPN access. Back To All My Macs. ; )
For the record, I have the following ports open on my Airport Extreme to allow L2TP:
500 - IKE
4500 - NAT Transversal (This is the one that was conflicting with BTMM)
1701 - L2TP Traffic (PPTP would require a separate port)
None of these ports are spelled out in the Port Mapping pop-up menu, you need to enter them manually and point the services to your VPN server.
Good luck to you all...
Message was edited by: danpoarch