The KDE team is shipping its first beta for the next-generation KDE desktop. The 4.0 beta is the starting point for the integration process to start bringing new technologies in KDE 4 libraries to applications.
Cautious users might consider this more of a late alpha release than a beta. The codebase is "still moving quickly" according to the release, and a quick review of the openSUSE live CD shows that KDE 4 isn't really ready for day to day use -- even for testing.
However, KDE 4 is now in library and application freeze -- the libraries should remain stable and no new applications are going to be allowed in. New application features will be allowed in, but the release is starting to gel into something approaching the final 4.0 release.
According to the release notes, integration between Dolphin and Konqueror is improving, Konsole has added support for different colors between tabs and clickable URLs, and Konsole will now provide color hints to applications like Vim. This release also includes KRDC, a remote desktop tool for administrators.
If you're interested in developing for or contributing to KDE 4.0, source code is available from the KDE team, and several distributions are making precompiled packages available, including Kubuntu, openSUSE, and Mandriva.