The latest version of VirtualBox (2.1.0) sports a preliminary accelerated 3D graphics layer which enables the virtual machines to have accelerated 3D graphics. Something similar has been available for VMWare and Parallels but this has now become a reality for the best 'freeware' virtual machine out there - in my humble opinion.

The way acceleration is achieved is through OpenGL which is a good industry standard - although using this layer on Vista might cause problems as a lot of Vista users are not able to use OpenGL in programs when the Aero interface is enabled. I am one of those users and I have to switch to the 'classic Windows theme' in order to get OpenGL programs to display something. With Aero I just get an empty 'clear' window where the borders and widgets are all there but the rest of the window is just totally transparent.
All the guest OS needs is to install an OpenGL driver that will run with the virtual hardware device and calls will be tunnelled to the host's GPU.

Anyways I haven't tested out the 3D emulation yet but this could mean a slight step into the direction of running older non-XP/Vista-compatible 3D games on a Vista/XP machine.

More info will follow shortly.