VirtualBox has received a major update in version 2.0. The user interface has been converted from Qt3 to the nicer-looking Qt4, 64-bit guests are supported (with a 64-bit host), and there have been major performance improvements. See the changelog for all the details.
If you’re currently using VirtualBox from the Ubuntu repositories, remove the old version and modules first:
sudo apt-get remove virtualbox-ose virtualbox-ose-modules-*
Then download and install VirtualBox 2.0 from the DEB package provided on the Linux downloads page. VirtualBox will be added to the applications menu under System Tools->Sun xVM VirtualBox.
If you’re getting the error VirtualBox can’t operate in VMX root mode when starting a VM, here’s what to do. KVM is blocking VirtualBox, so unload the KVM module:
sudo modprobe -r kvm_intel
This version of VirtualBox still does not work with the Ubuntu 8.10 alphas unless you enable hardware virtualization (see the last section of this post).
[update] As some commenters have reported, VirtualBox 2.0 doesn’t work with QGTKStyle.
[update 2] VirtualBox 2.0.2 is now out, and fixes issues with Ubuntu 8.10 and QGTKStyle.