The short answer is: yes, of course -- we've done it for years.
However, don't get too spooked. The stuff that we are able to read has so far been extremely limited.
Consider the book as a metaphor for a brain. Until recently, we've only been able to read the shape of the book. However, functional magnetic resonance imaging (fMRI) has advanced to the point that we can now read some of the Cliff Notes of that book as well.
The latest issue of Neuron describes some fascinating work by Miyawaki et al: Visual Image Reconstruction from Human Brain Activity using a Combination of Multiscale Local Image Decoder. In this study, the authors presented simple visual patterns to volunteers. The authors were then were able to reconstruct the patterns seen from the volunteers' brains using fMRI. Check out the following reconstructed images:
As Michael Russell commented:
It looks like the JPEG compressor in the test subject's brain is set WAAAY too high.Agreed. Even so: Wow.
The keys to this god-like power are topological maps of the cerebral cortex, such as the well-known sensory and motor homunculi first mapped out by Wilder Penfield.
It turns out that there are also retinotopic maps, connecting retinal stimuli to certain areas in the visual cortex. By imaging functional information directly from volunteers' occipital lobes with fMRI, Miyawaki et al were able to use such retinotopic maps to reconstruct what the subjects saw.
At the moment, it takes a lot of time and equipment to pull off this kind of mind-reading. So, by the time your coworker realizes that you've been undressing them in your mind, you will have already put their clothes back on and decided that they look a lot better that way.
In any event, this procedure will probably only work on information from the sensory and visual cortices, i.e. stuff that you are currently feeling or seeing. It won't work with stuff you are thinking or remembering.
No matter how many medical and legal uses we find for this technology, adolescent males of all ages will immediately leap to more prurient applications. What could horny geeks do with direct visual and sensory recording technology? The internet porn industry immediately comes to mind (so to speak).
For the moment it will be fairly easy to spot fMRI voyeurs -- just look for someone aiming a 7 foot ** roll of toilet paper at your skull. So, until this technology becomes a lot more portable, I won't be guarding my thoughts too closely. Sadly (or gladly), it will be a long time before nerds with iPhones can pull this off. However, that won't stop us from fantasizing about it...
** the approximate diameter of your typical fMRI scanner