lars's comment (http://news.ycombinator.com/item?id=3371968) is right on target. I recently finished my PhD in biomedical engineering, and the hot field that everyone wants to go into is what we're calling BMI - Brain-Machine Interfaces. The trick is, there are very few types of signals than can be reliably determined from these brain-signal reading devices.
Broadly speaking, there are two kinds of tasks that can be easily accomplished; anything involving moving limbs, or simple, low degree of freedom tasks (like moving a computer cursor). After months and months of training, a person can be trained to manipulate numerous degrees with pretty good reliability (i.e., move a robotic arm, AND control the mechanical pincer at the end), but this type of work doesn't generalize to other types of thought. We're nowhere near being able to extract sentences or words or being able to determine what complex scene is being viewed simply using brain activity patterns.
Broadly speaking, there are two kinds of tasks that can be easily accomplished; anything involving moving limbs, or simple, low degree of freedom tasks (like moving a computer cursor). After months and months of training, a person can be trained to manipulate numerous degrees with pretty good reliability (i.e., move a robotic arm, AND control the mechanical pincer at the end), but this type of work doesn't generalize to other types of thought. We're nowhere near being able to extract sentences or words or being able to determine what complex scene is being viewed simply using brain activity patterns.