As promised, KDE team has released KDE 3.5.9 as an update to their 3.5.x series which is considered stable for most users compared to early release of KDE 4.0. In this release, the focus of the updates are in KDE-PIM packages, since KDE team has merged the KDE-PIM enterprise branch, which has quite a lot features for an update packages. You can see the improvements on the KDE techbase.
Besides this updates, many other bug fixes and improvements have been made and it's all written on the detailed Changelog. I can say that the next Slackware release (12.1) will surely use this version, so stay tune on -Current as you will see this updates soon enough.