VirtualBox 3.1.6 works better with Slackware/KDE
The latest release of VirtualBox (3.1.6) corrects most of the problems that I had with Slackware and KDE. If you have been avoiding VirtualBox or KDE because of those issues you might want to take another look.
The only problem that I'm having now is that I have to click somewhere on the desktop to get a proper display when switching VirtualBox into "seamless" mode. The rest of the window display issues appear to be fixed and full screen mode also works correctly.