I'm using Gargoyle 1.3.14 with following setup:
1. ISP
2. ADSL modem
3. Gateway - DIR-655, WAN=PPOE, LAN=192.168.48.1 (DHCP server enabled)
4. Gateway - Gargoyle 1.3.14 on WRT-54GL v1.1, WAN=192.168.48.2 (static)
After exactly 24 hours the WAN IP of the Gargoyle router changes from 192.168.48.2 to 192.168.48.188.
Looking at the DIR-655, I see that the Gargoyle router requested a DHCP address and has been assigned an arbitrary one by the upstream router (which happens to be 192.168.48.188 here).
The new dynamic WAN IP is confirmed by ifconfig and iptables output. Within the GUI, Connection, Basic still shows the initial set up (192.168.48.2, Static IP wired).
The WAN IP change does not appear to have any adverse effect on operation other than causing slight horror (unable to reach GUI at known IP within 192.168.48.* subnet) and confusion. I didn't dare to change any configuration setting as yet.