Root-tail is a neat little app that prints tail-ed logfile output straight onto your root x window, 
with colours/geometry of your choice. It operates just like 'tail -f'. 

However, by default it doesn't seem to work on XFCE - you need to explicitely specify the ID of the 
display to use it (by using the -id flag of root-tail).I use the xprop command to find the active root 
display, and run this through the cut command to strip out the ID number for insertion into the root-tail `-id` field.

It looks a little something like this: (Using the messages/security log display setup from the manpage)

#: root-tail -g 800x250+100+50 -font fixed /var/log/messages,green /var/log/secure,red,'ALERT' -id `xprop -root XFCE_DESKTOP_WINDOW | cut --delimiter=' ' -f5` &

The important bit here being the -id `xprop -root XFCE_DESKTOP_WINDOW | cut --delimiter=' ' -f5` section, 
which obtains the active root display from xprop, and inserts it into the root-tail call. With this bit added, root-tail works fine on XFCE.