This happens on almost freshly installed Fedora 9. PackageKit Update Applet
0.1.12 icon appears shortly after boot and says "There are 62 updates available"
(in tooltip). I press on it and select "Update system now". The error message
box appears with the following text:

Failed to update system
The error was:
org.freedesktop.packagekit.update-system auth_admin_keep_always

If I run System -> Administration -> Update System program (IOW:
gpk-update-viewer), I see the list of updates available but clicking on [Update
system] button does nothing.

I am attaching the result of 

strace -tt -f -s 999 -o gpk-update-viewer.strace.log gpk-update-viewer

In the log, the moment I press [Update system] button is after this poll() call
- I waited for several seconds before pressing it in order to make it easier to
find it in strace:

...
3351  13:27:30.721970 poll(...
3351  13:27:36.706606 read(7, ....
...

Hope this helps.