Those seem correct. On further examination, this issue is caused by the system tray applets you have tried not supporting the latest "StatusNotifier" D-Bus system tray specification, which has also been adopted by GNOME.

In a KDE Environment, kded4 provides the "StatusNotifierWatcher" functionality, meaning that the applications register with that, and disable their legacy X.org system tray support in the process. Since none of the Xmonad system tray applets implement a StatusNotifierHost, the applications are not seen.

This is not usually expected under a KDE environment, and your fix is the only one possible at this time.

Thanks for the explanation. Of course the most ideal solution would be to simply use the Plasma tray, but it's hard to integrate cleanly as Plasma isn't very conducive to a non-desktop-metaphor model. If you check out some typical Xmonad/Xmobar screenshots you can sort of see the difficulty http://haskell.org/haskellwiki/Xmonad/Screenshots. Other tiling window managers likely have the same problem.

Generally Xmobar or a similar monitor is used to pipe in the current window title, tiling mode, and virtual desktop status - a very handy thing to have. Most people stick the tray next to that, but Plasma tray has a relatively large minimum size before it starts truncating the icons. Also the size and location of the Plasma tray is visually selected rather than scripted, so it can't be automatically made to precisely fit an adjacent monitor to the pixel. Other trays don't have these limitations.

Windows do respond to hints to get out of the way if you stick the Plasma tray into a panel on a screen edge, however if you switch into a fullscreen tiling mode it will remain on top unless the panel is set to allow windows to go over it. This of course makes the window hints go away so windows always go on top.

There's also the problem of the theme being very plasma-centric but I'm sure that can be adjusted. Overall it's easier to just not use plasma, but unfortunately the tray is one of the most important features of the desktop and is hard to live without. It would be really nice if Plasma's tray could be run in it's own decoration-less window like any other applet such as Conky or Dzen2, with command-line switches to control positioning and style. This would make it much more universally useful.

Do you happen to know whether disabling the StatusNotifier causes any other side-effects/loss of functionality when running KDE?

I am not aware of any. It would be completely possible to write a StatusNotifier compliant applet for XMonad however, as it is simply a D-Bus based freedesktop.org specification.