In the Draft version 1.4.draft-2 of the Extended Window Manager Hints (EWMH) spec
(http://standards.freedesktop.org/wm-spec/wm-spec-1.4.html, last updated on Sep 29, 2006)
there have been introduced two new types (among others) to the _NET_WM_WINDOW_TYPE
property:

_NET_WM_WINDOW_TYPE_TOOLTIP
    Indicates that the window in question is a tooltip, ie., a short piece of
    explanatory text that typically appear after the mouse cursor hovers over an
    object for a while. This property is typically used on override-redirect windows.

_NET_WM_WINDOW_TYPE_NOTIFICATION
    Indicates a notification. An example of a notification would be a bubble appearing
    with informative text such as "Your laptop is running out of power" etc. This
    property is typically used on override-redirect windows.

Here's an extract from the wm-spec-list mailing list
(http://www.mail-archive.com/###@###.###/msg00334.html):

    "A compositing manager deals with all windows that are visible on the
    screen, even override-redirect ones. This means new window types
    are needed so the compositing manager will know how to decorate the
    override-redirect windows with translucency and shadows etc."

So, we should probably add these new hints to the TrayIcon's tooltip and balloon
when the new spec is finilized.