Tomboy is very handy. It is an indispensable note-taking applet when I work on Gnome2 and I wish I could use it on Xfce too. It is available on other desktop environments as panel applet and it should be available on Xfce too.
They or someone else should write a panel plugin or tray support for this, not something for the panel.
I'm not sure if I expressed myself correctly. In Gnome2 I click on panel and choose option "add to panel" and just choose Tomboy and the Tomboy's icon is always on the panel. I wish the same was possible on Xfce. I installed Xfce on Ubuntu 11.04. In Gnome2 I have Tomboy available on the panel and when I relog to Xfce session I don't have Tomboy. That's why I ask for Tomboy because I' like to have the same functionality.
Yes, to have that in Xfce, someone needs to write a plugin, just like they did in Gnome.