Linux Photo Tools (The H)

Face recognition is a much more difficult problem than tracking, and is generally known as cognition.

First, the image should be registered (you may avoid this if you have a very good quality image with enough resolution and the person has not moved her face in an awkward angle)
Second, features are extracted (to reduce the dimension); you may want to get some heuristic approach for this, or rely on a statistical approaches or on some eigenvalue decomposition, or wavelet transformed (thus reduced) feature space, or a combination.
Third, you have to perform steps 1 and 2 for a set of people you are interested in, and record the features in a database
Fourth, when you have a test image (to recognize the face) you can again perform the first two steps then compare the result against your database.
If your feature extraction is hierarchical with low-to-high (fast-to-slow, coarse-to-fine) approach then recognition may be fast enough to be practical.

Now, with OpenCV you can do all of the above!
And if you have an Intel CPU you can get the best from OpenCV using IPP.