I am using an Dell Latitude D830 with Ubuntu 12.04.
Now, when I turn off the pc, Ubuntu doesn't halt completely sometimes.
I think that is due to the latest Nvidia drivers, because in the past with Ubuntu 10.10 I installed the latest drivers released by nvidia and I had the same problem.
By now, "messages" file / var / log is always empty.
My video card is: NVIDIA Corporation G86M [Quadro NVS 135M]
PC will not turn off even with "halt -p".