We've just brought a Pix 506E and set up the VPN using PDM.
Bog simple config - enable front interface and a back interface, VPN pool from the back interface range (IP address of rear interface =10.10.129.4, address pool = 10.10.129.100 - 10.10.129.150). Using local authentication.
I can connect two clients - after that no further clients can connect. I'm missing something stupid like licencing, I know, I did spend most of yesterday afternoon doing the google without success (too much information mostly), so I'm sorry but I'm now bothering you the more experienced, for any more clues....