@Heiko:

I just installed the Fedora 19 Alpha in VirtualBox, downloaded pyupdatesd and then modified its configuration section to change the "interval" from 900 seconds down to 10 (to get quicker feedback on whether the script is working or not). It worked for me. After about 10 seconds from running the script, the tray icon and message appeared, and clicking it brought me to the yumex GUI.

This script waits at least until the interval is first reached (i.e. 15 minutes after the script is run, in the default configuration), before it runs `yum check-update` to see if there are updates. When there are, `yum check-update` exits with status 100, and that's how the script knows when to show the icon and notification. If this isn't working, can you try debugging it? Around this part of the code, you can have it print the status and output of check-update:
    if int(time()) >= next_check:
        status, output = commands.getstatusoutput(c['check'])
        status = status >> 8

+       print "status:", status
+       print "output:", output

        # Updates?
        if status == 100:
Also the results of running, in bash: `yum check-update; echo $?`

@Frank:

Make sure you're running the script from within X11 (so that $DISPLAY is set). I see that error message if I attempt to run the script from text mode. ;) In my XFCE desktop, adding the script to my session startup list works for me.
$ cat ~/.config/autostart/Update\ Daemon.desktop
[Desktop Entry]
Encoding=UTF-8
Version=0.9.4
Type=Application
Name=Update Daemon
Comment=
Exec=/home/kirsle/bin/pyupdatesd
OnlyShowIn=XFCE;
StartupNotify=false
Terminal=false
Hidden=false
-Noah

On 05/26/2013 05:00 AM, xfce-request@lists.fedoraproject.org wrote:
----------------------------------------------------------------------

Message: 1
Date: Sun, 26 May 2013 09:37:52 +0200
From: Heiko Adams <heiko.adams@gmail.com>
To: Fedora Xfce users and developers <xfce@lists.fedoraproject.org>
Subject: Re: pyupdatesd: A Yum Updates Tray Icon for XFCE
Message-ID: <51A1BBD0.2020707@gmail.com>
Content-Type: text/plain; charset=UTF-8
Hi,
it looks like your script doesn't work on Fedora 19. I've tested it this
morning and absolutely nothing happened. No errormessages but also no
notifications about new updates - but updates were available.


------------------------------

Message: 2
Date: Sun, 26 May 2013 09:41:10 +0100
From: Frank Murphy <frankly3d@gmail.com>
To: xfce@lists.fedoraproject.org
Subject: Re: pyupdatesd: A Yum Updates Tray Icon for XFCE
Message-ID: <51a1caa8.c248b40a.5742.ffffb486@mx.google.com>
Content-Type: text/plain; charset=US-ASCII

On Fri, 24 May 2013 14:15:50 -0700
Noah Petherbridge <root@kirsle.net> wrote:

Fedora 18.x86_64

traceback (most recent call last):
  File "/etc/sysconfig/pyupdatesd", line 55, in <module>
    import gtk
  File "/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py",
line 64, in <module> _init()
  File "/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py",
line 52, in _init _gtk.init_check()
RuntimeError: could not open display

-- Regards, Frank - I check for new mail app. 20min www.frankly3d.com