I found out about the notion of "The Uncanny Valley" while reading this article (popular science) about a guy who built a realistic robotic head as a result of a pickup line for his now girlfriend/fiance ("Can I make you into a robot?").

The Uncanny Valley was conceptualized by Japanese roboticist Masahiro Mori, who argued that initially, as you make a robot more human-like, the emotional response of humans to it increases. At a certain point, though, if you make the robot more human-like, the emotional response plummets into a sharp negative reaction. Only when the robot approaches nearly perfect simulation does the curve rise once more.
- More on the Uncanny Valley

Update (04/20/04): Uncanny Valley example