iTunes is not working for me now either (10.8.3 with iTunes 11.0.2). It was working not very long ago, but I can't recall if that was before or after 10.8.3 so it is unclear what broke playing protected content. Unprotected videos play fine. I don't see any messages in the Console logs, so no help there so far.
I tried removing my 650 Ti graphics card and used the HD4000 graphics. I had forgotten to change GraphicsEnabler (left it at "No") and I was surprised to find that trying to play videos now, I could hear the audio but there was no video, and it was the same for protected and unprotected videos in iTunes. I expect that was because QE was not enabled (no ripples in Dashboard).
When I rebooted with GE=Yes with HD 4000, (QE now enabled) I was back to being able to play unprotected videos, but nothing (no audio or video) from protected videos. I now had the Airplay icon in my menubar (odd since I use MacPro 3,1). Since there was no improvement in videos, I replaced my 650 Ti card, so still can't play iTunes protected videos. I can live with it for now (I have an Apple TV and an iPad to play videos on), but I would like to find out how to fix this.
I have not yet tried the edits to enable both the 650 Ti and HD 4000- given the above experience, I am not so sure it would work. Has anyone found this approach to work?