Unlike GNOME’s panels and panel applets, KDE 4.0’s plasma widgets don’t have to be placed in a panel. In fact, you don’t need to have a panel at all. Here’s how to remove the bottom panel in KDE 4.0:
Browse to ~/.kde4/share/config/ and make a backup copy of of the plasma-appletsrc file. This is the file you will need to edit. You can use the backup to restore the panel and your widgets to their previous configuration.
Remove all your widgets from the panel. (I’m not sure this is necessary.)
Log in to another desktop environment. You can’t edit the plasma-appletsrc file while logged in to KDE 4.0 because your changes will be overwritten. I used GNOME to do the editing, if you don’t have another desktop environment to use you can log out and switch to a virtual terminal. (Use Ctrl-Alt-F1 to open the terminal and Ctrl-Alt-F7 to switch back.)
Open ~/.kde4/share/config/plasma-appletsrc. Find the section defining the panel and remove it. This is what it should look like:
Save the file, switch back to KDE, and the panel should be gone.
I added the Application Launcher Menu, Digital Clock, Notes, Pager, System Tray, and Task Manager widgets to the desktop to replace the panel. The only problem is the Task Manager window list, which is always a square.
When windows are on top of your widgets, press Ctrl-F12 to bring them to the top.
See Lifehacker’s Design Your Own Desktop with KDE 4 for more about using KDE with a taskbar-less desktop.