VirtualBox 4.0.0 is a major update and includes the following major new features:
- Reorganization of VirtualBox into a base package and Extension Packs
- New settings/disk file layout for VM portability
- Major rework of the GUI (now called “VirtualBox Manager”):
- Redesigned user interface with guest window preview (also for screenshots)
- New virtual hardware Intel ICH9 chipset with three PCI buses, PCI Express and Message Signaled Interrupts (MSI)
- Intel HD Audio, for better support of modern guests
According to LifeHacker you can even use VirtualBox to run a Mac OS-X guest !