I've wanted to have XMMS run as a notification area (system-tray) only application since I first tried it out, but as most of you probably know, that wasn't possible. Until now, that is. I've discovered that KDE 3.3 makes it quite easy to have XMMS (or any other app for that matter) hidden from the task bar. Then, with the status docklet plugin, you can access it from the system tray only. I know I'm not the only one who wanted this, so I'm going to tell you all how to do it.
First, start up XMMS normally. Configure the status docklet plugin the way you like. I suggest setting one button as a "toggle all windows" button so that you can easily pop the main window on and off once the task bar button is gone. Make sure it displays normally in your system tray. You might also want to toggle the "always on top" setting as well to keep it from being hidden by other windows when you pop it up.
Next, open up the KDE control panel and switch to the Desktop tab. There you'll find an entry for "Window-Specific Settings". That's what we want. Create a new entry and click on the "detect" button on the first tab that comes up. Your pointer will turn into a crosshair. Now click on the XMMS window. An information window will pop up. Make sure "Use window class (whole application)" is selected and hit ok. The window info will be entered into the settings.
We're almost done. Switch to the "Preferences" tab and enable the entry for "skip taskbar". In the dropdown next to it select "Apply Initially" and checkmark the box on the right. Now simply save your settings and restart XMMS. It should launch without appearing in the task bar, but you'll still be able to pop it up through the docklet plugin. Enjoy!
BTW, remember that this is for KDE 3.3 only. But for those using older versions or for Gnome users there is something you can use to get almost the same effect. There's a small applet called KDocker
that will force apps into the notification area. You'll probably have to compile it from source, but that's not too hard. Then you can simply run "kdocker xmms" and XMMS will be reduced to a tray icon. It's not quite as good as it will only toggle the main window on and off and doesn't have any menu controls, but you can use the docklet plugin next to it if you want that kind of control. The main thing is, it's off of your task bar.