Kubuntu won't include KDE 4 as the default desktop for at least two releases: "This is due to the fact that the next version of Kubuntu (7.10 'Gutsy Gibbon'), will not include KDE 4 because it will not be ready by then. The version after Gutsy would be totally perfect for KDE 4, however, this version will be a Long Term Support (LTS) version. These specific Ubuntu versions are released every 18 - 24 months and aim at long term stability. Therefore they only include packages which are already in a pretty stable and reliable state. And Jonathan Riddell mentioned that he does not believe that KDE 4 will reach that state."
So yeah... the time is ripe for KDE Mint to take over the scene and become the number 1 KDE distro out there
