The most recent stable version of KDE Community Edition is based on Linux Mint 5 "Elyssa".
In terms of the KDE CE based on Linux Mint 6, this was said on the blog January 4, 2008
After a lot of testing and talking we decided KDE 4.1 wasn’t fit for usage and we couldn’t release it [KDE CE 6] this way. The decision was made to wait for KDE 4.2 stable (which is planned for the end of this month) and to then design a Mint 6 KDE CE based on Kubuntu 8.10 but with Amarok 2.0 and KDE 4.2. This means there won’t be any KDE CE release this month.