A Clue Bat was gently swung by a friendly and clueful (semi-anonymous) AOL NetOps guys who contacted me from my post on Nanog. Thanks Nanog, and this sounds strange from me, but Thank's AOL. :) And yes, it should have been obvious on my part.. a router was configured with a 172.0.0.0/8 netmask.
......there is what we call an RFC1918 issue. AOL was given some IPs in the 172.16.x.x range by ARIN. These are valid routable IPs, and we use them as IPs for the AOL user's machines (kinda like DHCP). The problem is that some people block all of 172.x.x.x thinking it's only for non-routable IPs when it's only half that range that is non-routable. (172.16.0.0/20 is the routable part). That appears to be the case with this one. We've asked ARIN for a different range, and they told us to go away, so we are stuck with this issue. If you can ask someone who does firewall and/or router ACLs in front of that website, they should be able to fix the issue.