The Virtualbox (non-free) repository is available for Ubuntu 10.4 Lucid Lynx (kind of late I'd say). You can add it using the 
following command: 

sudo add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian lucid non-free"
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -

Then install it: 

sudo apt-get update && sudo apt-get install virtualbox-3.2