EUR 10.-Upright, human like two-legged walking/locomotion: What are strategies to build robots that walk on two legs?

Upright, human like two-legged walking/locomotion: What are strategies to build robots that walk on two legs?

I am a new to the field of robotics. I have seen robots that walk on two legs such as:

• Nao: http://www.aldebaran-robotics.com/download/NaoAcademicsV3.mov
• Abert Hubo: http://upload.wikimedia.org/wikipedia/commons/6/66/Einstein-Hubo.jpg inspired by the famous Honda Asimo robot

Nao achieves pretty good walking skills, so may question then: what are challenges of tow-legged walking and how are they currently solved?

I imagine that a robot has a certain set or preferred parameters, and if distorted one way or the other, seeks to regain that set or parameters. Ideally, the body is designed that the mechanics counteract those distortions. Is that right? Then again it is unclear to me what the issues are in two-legged upright locomotion.