The move to re-architecting the Kontact PIM (Personal Information Management) applications on the Akonadi framework has taken years, but now it's ready for both desktops and mobile devices. The interfaces to Kontact may appear the same, but internally these applications are shaped very differently. Among the improvements are faster email notifications, improved IMAP email performance, and better interoperability between the PIM applications. The transition to Kontact 2 should be seamless for end-users.
There's also now Kontact Touch, which are the KDE Kontact applications setup for mobile / touch-screen devices. More information can be found in the KDE press release or the Kontact Touch video embedded below.
At the same time, KDE SC 4.6.4 is available as a monthly point release to deliver various translation updates and bug-fixes. The next major release, KDE SC 4.7, is planned for late July.