KDE recently released a beta test version of what will become KDE 4.6.0. Besides stability fixes this version seems to further improve the way Activities are handled and integrates Nepomuk into the Dolphin File Manager to achieve very interesting results. More info can be found here.
You should be able to download and what is actually version 4.5.80 of KDE. Note that this is a beta version and is likely to set your computer on fire and kidnap your children.
KDE 4.6.0 is to be released in January 2011 and will likely be the basis for Kubuntu 11.04.