Today, on January 27th, the KDE team announces the stable version of KDE 4.2.0, so we have moved all kde-packages from [testing] to [extra].
We suggest that you exit from KDE before you update, backup your ~/.kde4 configuration dir and start with a clean config.
Take a look at old plasmoids, they might not work with the new Plasma.
You can find the ChangeLog on: http://www.kde.org/announcements/4.2/
For any problems (we hope there are not) please use the bugtracker or the arch-general mailing list as usual.