My wife needed some new headshots, and I wanted to test my old 85mm f1.8D with the new camera, so we put these together the other day on a whim. I had the autofocus set to automatic area selection to see how well the face detection worked. It looks like on the first image, it focussed on her rear eye. But all in all, it did a great job.
For people photography, I think the auto area works great unless I want the focus to be super critical. Honestly, I'd rather keep it on auto face detect and spend my time connecting and directing the model/subject than think about putting the camera into single point focus and making sure the selected point is always over the eyeball.
Processed in Lightroom 4.1 RC2. The crops are 100%.