What are some face detection (not recognition) algorithms suitable for limited (embedded) processors?
We know that the problem is solved because our cell phones and digital cameras can do it. You might be able to find out what algorithm they use by trawling the patent databases.

Computer vision libraries like OpenCV and SimpleCV offer face detection as a standard feature. See for example Near realtime face detection on the iPhone w/ OpenCV port. If you want to DIY see Simple Face-detection Algorithm Based on Minimum Facial Features.