As announced last year* and after a longer period of testing, QT versions 3 and 4 have now been moved to [extra].
At least until the final release of KDE4 there are plenty of packages which still use version 3 of QT. Therefore a new package called qt3 was introduced. The dependencies of all packages are updated according to this change. The update should be quite smooth. Pacman only notifies about replacing qt by qt3 and qt4 by qt.
If you don't want to have QT version 4 installed just remove it with "pacman -Rc qt" after the update. If you are using any third party repositories like KDEmod please wait until they have updated their packages, too. Do not force any updates! If there are still any issues, please report them at http://bugs.archlinux.org.
Community packages may not be updated with the new versioning scheme of QT, so give your TUs some time to make the required changes.
*) http://archlinux.org/pipermail/arch-dev-public/2007-November/002910.html