There were some warnings are errors in the log file for chat, particularly one about the owner.key not being found in (eg) /activities/uid_to_home/1001 (etc), and after fighting with that for a bit I disabled Rainbow ( mv /etc/olpc-security /root ; reboot ) and that eliminated the errors from the log but it still does not detect any one other than me.
I think the double NAT is causing the trouble, but I haven't pinned it down yet, sorry.
http://dev.laptop.org/ticket/5083#comment:1