Very interesting stuff. The one achillies heel I see for this is, it only works if you pre-train the computer with the brain patterns of many people looking at the given object. If you never trained the computer to recognize what the brain activity for a unicorn looks like, and the person being scanned thinks of a unicorn, then the computer is going to have no idea what's going on.
This will get interesting, however, when we find a way to take all this stimulus data from the brain of one person and stimulate the exact same regions in the brain of another person. Then you'll have something like telepathy. A crude kind of Deanna Troy psychic empathy - you feel similar kinds of sensations and emotions as the other person does, because similar parts of your brain and their brain are being stimulated.