xfce4 from testing, which can only be installed via pacman -Sf xfce4 due to file conflicts (as reported already on the bug tracker), has problems with the systray.

When adding the systray to the panel, I get an error message "Could not open module 'systray'".

My installed xfce4 packages are:

[nv@rattlesnake ~]$ pacman -Qs xfce|grep local
local/exo 0.3.1.12rc2-2
local/gtk-xfce-engine 2.3.99.2-1
local/libxfce4mcs 4.3.99.2-1
local/libxfce4util 4.3.99.2-1
local/libxfcegui4 4.3.99.2-1
local/mousepad 0.2.10-1
local/terminal 0.2.5.8rc2-1
local/thunar 0.5.0rc2-1
local/xfcalendar 4.2.3-1
local/xfce-mcs-manager 4.3.99.2-1
local/xfce-mcs-plugins 4.3.99.2-2
local/xfce-utils 4.3.99.2-1
local/xfce4-appfinder 4.3.99.2-1
local/xfce4-dev-tools 4.3.99.2-1
local/xfce4-icon-theme 4.3.99.2-2
local/xfce4-iconbox 4.2.3-1
local/xfce4-mixer 4.3.99.2-1
local/xfce4-panel 4.3.99.2-1
local/xfce4-session 4.3.99.2-1
local/xfce4-systray 4.2.3-1
local/xfce4-toys 4.2.3-1
local/xfce4-trigger-launcher 4.2.3-1
local/xfdesktop 4.3.99.2-1
local/xffm 4.2.3-1
local/xfprint 4.3.99.2-1
local/xfwm4 4.3.99.2-2
local/xfwm4-themes 4.3.99.2-1

The same bug has been confirmed on the mailing list:

http://bbs.archlinux.org/viewtopic.php?t=28594&highlight=xfce