Take the tour ×  I updated Ubuntu 12.04 yesterday (09/01/2013).
After the update (updated the kernel, and several other things
that I noticed), it's requested a system reboot.  Rebooted:
Unity 3D is loaded (only the vertical bar), the "Unity
background is purple" and nothing else is loaded icons, top bar,
etc. (CTRL+ALT+F1 doesn't work, it's completed freeze).  Only
works the Unity 2D or XFCE.   ➜  ~  sudo glxspheres  Polygons in
scene: 62464 Visual ID of window: 0xa6 Context is Direct OpenGL
Renderer: Mesa DRI Intel(R) Sandybridge Mobile  60.171477
frames/sec - 67.151368 Mpixels/sec 60.013886 frames/sec -
66.975496 Mpixels/sec 59.871287 frames/sec - 66.816357
Mpixels/sec ➜  ~  sudo optirun glxspheres Polygons in scene:
62464 Visual ID of window: 0x21 Context is Direct OpenGL
Renderer: GeForce GT 540M/PCIe/SSE2 94.500831 frames/sec -
105.462928 Mpixels/sec 96.664706 frames/sec - 107.877811
Mpixels/sec 96.986230 frames/sec - 108.236633 Mpixels/sec
107.303790 frames/sec - 119.751029 Mpixels/sec ➜  ~    OS: Ubu