One can reverse engineer the signals inside a point and shoot camera (the one with face detection feature). when it starts to draw squares around faces, Arduino should pick the biggest face and follow it.
Another way is to find the datasheet of the IC used for face detection inside these camera's and use them.
Just wanted to stir some idea's if anyone wants to elaborate please.
By your comments, I am almost certain that you have never opened up one of these cameras; had you done so, you would have realized the absurdity of what you wrote.
Long story short is that you are unlikely to find any specific "signals" to reverse engineer; at best you might be able to find some kind of I2C or SPI interface, or some kind of other proprietary programming interface - which may be useful, or it may not be.
Furthermore, you're not likely going to find a datasheet for the IC used; if it isn't a house-numbered part, it will likely be some proprietary ROM-masked microcontroller or ASIC that will be all but impossible to reverse engineer (given enough money, time, and tools - anything can be reverse engineered, but the cost can become way larger than it is worth).
Stick with OpenCV on this one; trying to hack any singular point-n-shoot camera with face detection is likely to be a fruitless pursuit at best.