Christian, a leak is certainly possible depending on the version and distro you are running. openSUSE 10.2 had a leak that you need to run the updater for. At one point a big problem was also hal libraries leaking (and when you have to poll, this adds up).
My mentioning of Novell was misleading here.
I just was really surprised that it was the main-menu that slowed down my system that much by consuming soooo much memory! 
I wouldn’t be surprised if that is already fixed.