The properties dialog sets itself to be always on top. This is probably fine, but it seems like other panel apps (battery monitor, xfce4-mixer and net monitor to name a few) do not set their properties boxes to be always on top. It should be uniform - not sure if there is a xfce4 "constitution" you can refer to for this...
There's a fairly abandoned set of human interface guidelines here: http://wiki.xfce.org/panel-hig
However, these say nothing of whether the dialog should be always on top.
There are examples of other plugins that set the dialog always on top, like the pager.
Personally, I think that, since the dialog window does have a task list entry, it should not be always on top.
Jannis, your thoughts?
Fixed in r7429.