After many release canidates and a lot of testing. KDE 3.1 has finally gone final. KDE 3.1 comes in two forms. The first is a binary (if one is available for your OS). A binary is a precompiled package to install on your machine. The second method of installing it is to compile the source yourself. Right now there are binaries available for ASPLinux, Connectiva, Debain, SuSE, Slackware, and a few other OSes. There are many updates and new features in this version. KDE has not officially announced its release so you are hearing it here first. I will update you with a full list of changes as soon as KDE makes them available.
Update: Added KDE 3.1.0 Changelog