Following previous posts about installing Virtualbox, here’s the 3.0 update (currently 3.0.8).
If you have a previous version already installed you need to manually install the new one.
Install
- Go to System > Administration > Software Sources and Third-Party Software.
 - Add the following sources for Intrepid (check your distribution here)
 - Download and register Sun authentication key
 
wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -
- Be sure your source list is updated
 
sudo apt-get update
- Be sure you have dkms installed (so Virtualbox host kernel is automatically updated as you update your system kernel)
 
sudo apt-get install dkms
- Install Virtualbox 3.0
 
sudo apt-get install virtualbox-3.0
- Logoff and login again
 - Start Virtualbox at Applications > System Tools > Sun xVM VirtualBox
 - If you want you can optionally register your VirtualBox installation
 - If you have a Windows Virtual Machine don’t forget to install the Guest Additions (after starting the virtual machine go to Devices > Install Guest Additions).