One of the myriad new features in Ice Cream Sandwich that got people really excited about Android 4.0 was Face Unlock, an alternative to sliding, PIN or pattern-based screen unlocks that uses a forward-facing camera to literally recognize the user's face. Upon actual use, however, it's clear that the technology isn't quite as fleshed out as it could be: frequent errors necessitate a backup system for most users. Apparently some users in particular are having trouble: a Reddit user identifying himself as a black man said that he has consistent issues with Face Unlock not recognizing him. His problems are echoed by other users throughout the thread.
Let's start off by getting this right out of the way: Google isn't racist, or at the very least, problems with face detection don't make them so. Differing skin tones, particularly for those of African, Indian, Middle Eastern and Latin American descent, present problems for cameras of all kinds. Microsoft's Xbox Kinect peripheral has a lot of the same issues, as do other facial recognition systems. Unfortunately, detecting faces in areas of low contrast is still pretty difficult, even for modern camera sensors and software - it's more of a physical limitation of light refraction than anything else. The original poster said that the issue persists in both dark and overly lighted areas.
Face Recognition is more of a toy than a security feature, at least at the moment. As far as biometrics are concerned, fingerprint reading is still a much more secure method of logging in and/or unlocking a device - ICS's facial recognition can even be fooled by a well-printed photo. Here's hoping that Google can improve the software in future versions, for everyone's sake. Hey Apple, when you patent the tech that's already in Android, maybe you can make it "just work".