I wrote a little script to get the current public IP address from the Fonera status page. In my set up the box has the default internal IP address 192.168.10.1.
Here the full script:

#!/bin/sh

IP_ADDRESS=`wget -q -O - http://192.168.10.1/cgi-bin/status.sh | grep "IP Address" | sed -n '//{p;q;}' | sed 's/.*//' | sed 's///'`

# IP Address:88.130.197.125
if test "`cat /tmp/current-fonera-ip.txt`" = "$IP_ADDRESS"; then

  echo "not changed"

else

  echo "changed, update"

  # do something like dyndns update

  wget -q http://user:password@server/nic/update?hostname=hostname.domain.tld
  echo $IP_ADDRESS > /tmp/current-fonera-ip.txt

fi

(As you see I used it to do a dyndns compatible update. On the server backend this just requires a bind configuration and a simple update script. Maybe I will show that later here.)