This first point release to the KDE SC 4.5 series brings fixes to some platform components previously plagued by stability issues (largely when using the new shared data cache), improved ergonomics of moving applets inside Plasma panels, listing of UPnP devices in the network KIO slave, KWin now works in more configurations, and the weather and news applets now update their data properly when a network connection is restored.
A list of other changes for KDE SC 4.5.1 can be found in the change-log page.
KDE SC 4.5.2 is expected to be released on the 30th of September. The next major release of this desktop environment is KDE SC 4.6 and it's expected to be out towards the end of January barring any delays.
The KDE SC 4.5.1 release announcement can be read at KDE.org.