The only camera + processing + software that is accessible to a hobbiest to recognize faces (that is, to recognize a face from a soccer ball, not between two different faces) is the CMU Cam 3 and maybe the POB Camera. The crucial criteria is that it is cheap, and the complex camera interfacing, memory buffers, and processing are all done onboard for you. THe second criteria that separates these two cameras from the others (mostly older versions of CMU Cams, there aren't many out there for hobbiests) is that you can have customized firmware for your own algorithms.
I believe if you google there is someone who has code to make those cameras recognize faces. I think it was for the CMU Cam 3.
Any other method either costs a lot more money (like buying a camera + processing package from a company and thus will cost you an upwards of $5000), or a lot of money and time (the development tools for processors able to deal with image processing are an upwards of $3000, not to mention the cost for parts).
POB Cam:
http://www.pob-technology.com/
CMU Cam in general:
The CMUcam Vision Sensors
CMU Cam 3 (and the face recog stuff):
**broken link removed**
THe rich hobbiest route:
SR3000 Swiss Ranger Camera
**broken link removed**
THe professional, but lazy, route:
Point Grey Research Inc. - Home
Although, if you can afford either of the last two solutions, I would instead spend half as much money and forego on the face recognition in exchange for VISUAL NAVIGATION and buy something from these guys:
Welcome to Centeye