Wrote a quick and dirty script to scan the listener log for incoming connections and produce a count of how many connections came from which host. Here is the URL:
All it does is hack out the ip address from the second HOST= entry of the lines in the listener log that have HOST= in them. Then it uses the Python function socket.gethostbyaddr to get the hostname for those that can be looked up this way.
Counts the number of lines for a given ip address and sorts them from most to least connections. Might be useful to someone.