The problem appears to be a bug in the RG's filtering firewall that blocks the returning ICMP messages that traceroute uses to identify intermediate hops even when all firewalling is supposedly turned off. When full logging is turned on in the RG, the following incriminating messages appear in its log:
Jan 8 15:41:09 uverse 126.96.36.199: INF 2010-01-08T15:41:06-08:00 fw,fwmon: src=188.8.131.52 dst=184.108.40.206 ipprot=1 icmp_type=11 icmp_code=0 ICMP Packet Error, Dropped
ipprot 1 is ICMP; icmp type 11 is Time Exceeded; icmp code 0 (when icmp type is 11) is TTL expired in transit.
This is a good example of the kinds of problems that could be so easily avoided if the Residential Gateway were to act as a simple transparent bridge for all static IP traffic.