I wrote a little Perl script which beeps and uses libnotify to show you when the leak is occurring (attached, argument is number of seconds between checks). I get a leak of about 4kb every 5 seconds, although there are periods of 20 seconds about once a minute where I get no leak.
It seems like the leak only happens when knetworkmanager is running. When I close knetworkmanager, the leak stops.
I found the best way to re-start network manager once it had grown too memory-hoggingly massive, was to do "sudo /etc/init.d/dbus restart". It will drop any wireless connections when you do this though. No idea what it does to Gnome and/or KDE - I've become addicted to fluxbox of late :-)