Today, we are passing the last milestone on the way to KDE 4.1, a release that will be suitable for a larger audience than 4.0 has been. While it is not yet up to the features that people are used to from KDE 3.5, KDE 4.1 provides a significant amount of improvements over KDE 4.0, which some said was a bit of a bumpy ride. Sources and available packages are linked on the release info page.
KDE 4.1-rc1 is the only release candidate for KDE 4.1, which will be released on July 29th.
The development in trunk/ in Subversion has already been opened for feature development, which is going into KDE 4.2 (to be released in January), but developers are strongly encouraged to concentrate on bugfixing in the 4.1 branch for now. Do give RC1 a spin, file bugreports and fix things, there is only a week left until 4.1 is being tagged. Do have your changes in the 4.1 branch reviewed by your peers, though. Note that some users might still be suffering from performance problems with NVidia graphics chips. There is a page on Techbase that gives some more information about it. Make sure you report bugs via KDE's Bugzilla so they can be addressed and do not get lost.