Robots are getting better and cheaper all the time. Sure, they're not perfect, but then neither are humans (and we, initially, design and build the robots). Some day, though, robots might start improving upon themselves at a rate that outpaces our human capacity. It could be a huge benefit to civilization to be able to replace all dangerous labor with robotic slaves, but some folks are worried about what the world will look like when robots are really that advanced. Here are just a few links on the scenarios of a robot-dominated age of technology.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post