I may have found it in the release notes ...
⢠CSCsj92778
Symptom After the WAN interface on the Modify Internet Connection dialog is disabled and reenabled, the PPPoE check box becomes enabled again even though a firewall configuration is present. Because of this problem, Cisco Configuration Assistant disables the PPPoE check box if it detects a FW/NAT configuration under FA0/0.
Conditions None.
Workaround To configure PPPoE, you must first delete the current FW/NAT configuration, then configure PPPoE, and finally reconfigure FW/NAT by choosing dialer0 as the outside interface.