For an example of one of the classic kinds of "debunking" that this kind of research tends to get, see "Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon:
An argument for multiple comparisons correction" ( http://prefrontal.org/files/posters/Bennett-Salmon-2009.pdf, PDF)
To add to Breakthrough's answer, the article states:
"The patients were repeatedly asked to imagine playing tennis or walking around their home. In healthy volunteers each produces a distinct pattern of activity, in the premotor cortex for the first task and the parahippocampal gyrus for the second. It allowed the researchers to put a series of yes or no questions to severely brain-injured patients. A minority were able to answer by using the power of thought."
So, I imagine they could say "To answer YES, imagine playing tennis. To answer NO, imagine walking around your home". Then they could ask yes/no questions and monitor the patterns of brain activity.
I have a friend who works at the research institute beside the university hospital, and he told me they basically ask a set of questions, while taking a series of measurements via the fMRI machine. They then compare the expected responses with the output from the brain scanner, and can match up the blood flow rates in different parts of the brain with a "positive" or "negative" response. They then perform the inquiries multiple times, until they have a high degree of probability that the questions were answered in a consistent manner.
I really hope you're right. The idea that someone in that sort of condition, apparently beyond further harm, could actually have been conscious for all those years, makes the average horror story look optimistic.
We don't know that such a person couldn't eventually enter an inner world where they find a kind of peace and fulfillment. It doesn't necessarily have to be a continual conscious horror of being frozen in a hospital bed.
What if they learn to meditate like a monk? Certainly there are people who can enter altered states of consciousness for extended periods who could perhaps tune out the torturous effects of any sensory deprivation.