This issue is documented in the Errata
Here's a cutout:
Sometimes after upgrading KDE4.2.x to KDE4.3.x you may find that most of the desktop wallpaper is black except for a very small part at the top, see this screenshot: https://bzattachment.mandriva.com/at...t.cgi?id=15113
There're reports that this goes away after logging out then logging back in, as the plasma configuration files get touched.
If it persists then this will definitely fix it. Run these commands as user (not root) in terminal:
kquitapp plasma-desktop rm -fv ~/.kde4/share/config/plasmarc plasma-desktop &