Here is how to get most of the 3D effects of KDE 4 working with a Slackware 13 Guest OS in VirtualBox 3.0.6.
First, create this small shell script or type in these commands.
# Set compositing type to "XRender". Click "Apply" on "All Effects" tab.
ln -s /usr/lib/modules/dri /usr/lib/dri
ln -s /usr/lib/VBoxOGL.so /usr/lib/xorg/modules/dri/vboxvideo_dri.so
The comment is just to remind you later about how to enable the effects in KDE.
Install the VirtualBox Guest Additions for Linux normally.
Run the shell script or type in the two "ln" commands.
Next, Click on "System Settings" in KDE 4.2. Double click the "Desktop" icon. On the "General" tab check the box next to "Enable desktop effects" but DO NOT CLICK "Apply" yet. Click the "Advanced" tab, and for the "Compositing type" select "XRender". Click the "All Effects" tab. Enable the effects that you want. I have not been able to get the Desktop Cube to work. You must click the "Apply" button on the "All Effects" tab (not other tabs).
Wobbly Windows seems way too slow to use but at least the fade effect seems to work well.
Any suggestions about how to improve the performance or enable more effects will be appreciated.
Credit to "geep" (Peter) and "JesperW" for posting the information in VirtualBox forums.