Code:
gksudo gedit /etc/apt/sources.list
add this on a new line at the bottom of the file.
Code:
deb http://download.virtualbox.org/virtualbox/debian karmic non-free
Code:
wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get install virtualbox-3.1
done.

im using the karmic virtualbox on lucid w/o problems