Update: The full-screen toolbar can be moved to the top of the screen, which (IMHO) is a much more usable location. To move it to the top of the screen, open the Settings dialog for any VM, select General->Advanced and tick the Mini Toolbar: Show at Top of Screen.
It looks as though the VirtualBox Team have added a nice new feature in their 3.0 release – the full-screen toolbar!
When running a VM in full-screen mode, moving your mouse to the bottom of the screen displays the new toolbar, as shown below.
All of the features available when running the VM in windowed mode are now available in full-screen mode, including: mounting devices, shared folders, shutdown options etc. The toolbar can also be pinned open if required.
For those Windows users reading this post, the toolbar behaves just like the Remote Desktop toolbar that is displayed at the top of the screen during a Terminal Services session.
This new feature is only available in the Sun VirtualBox 3.0 release and above. Download it now from VirtualBox.org.