With a patched version of VirtualBox, it's now possible to have accelerated Direct3D support within the guest virtual machine. This newly-introduced capability isn't limited to running atop a Windows host either. In other words, you can now run Direct3D-powered games within a virtualized Windows installation when on a Linux host operating system with VirtualBox.
This VirtualBox Direct3D support is dependent upon WineD3D for translating the Direct3D calls into OpenGL, which is then executed on the hardware. Converting all of the Direct3D functions to OpenGL, however, comes at the cost of some CPU overhead, but nothing more than what WINE consumes. A screenshot showing the first signs of success for this Direct3D support can be found on the VirtualBox web-site.
The announcement regarding Direct3D support for VirtualBox was made in this VirtualBox bug entry. The (fairly simple) patch that adds in this support can be found in this attachment.