Committed to the VirtualBox guest driver source-code in recent days is support for a KMS driver (e.g. this commit). It's part of the VirtualBox guest additions for the VirtuaBox Linux DRM component, but doesn't affect other platforms or make any other changes.
Oracle hasn't officially announced this new KMS driver for VirtualBox, but it's in the code-base and should be part of VirtualBox 4.2 Linux Guest Additions or whatever the next major release is dubbed.
Sadly there's no indications that they will push for mainline Linux kernel integration of this KMS driver, like their other guest additions kernel drivers, but is likely to just likely to keep shipping separately. Granted, many kernel developers feel the current VirtualBox drivers are tainted crap.
Hopefully Oracle also has some user-space 3D improvements for VirtualBox planned as the current OpenGL performance is rather poor.