Tomboy is a nice note taking application that I am using it everyday. But it is really annoying that no icon is there on the KDE tray. But it is really there, like a transparent icon. These problem seems a bug in the 0.4 release, because I saw someone else got the same question in some forums. But now with the 0.5.1 release, the tomboy icon finally can be displayed on the KDE tray. Of course there are other new features, please check here.
Tomboy icon on the KDE tray, finally