gpk-update-viewer, Fedora 13 --
The "Software Update" window comes up and at the top it says a new release of Fedora is available, and there's a button to click ... but you can't click it if gpk-update-viewer didn't find any updates for the packages in the current release, because there's immediately a dialog that comes up stating that there are no updates available ... the only option is to click ok, and it closes gpk-update-viewer entirely, never allowing the user to select the new release.
We moved the upstream bugtracker to GitHub a long time ago. If this issue still affects you please re-create the issue here: https://github.com/hughsie/PackageKit/issues
Sorry for the impersonal message, and fingers crossed your issue no longer happens. Thanks.