I have the impression that much of what is being done here is extremely heuristic. In fact, most people seem to apply this to the <120 chars of twitter statements. Probably the results (while not being computed this way) aren't much better than counting "positive" and "negative" words with a litte position information ("A better than B" = positive for A, negative for B)
When you then see companies buying a full twitter feed (that's how many mbit per second?) and claiming to do sentiment analysis on this this seriously makes me wonder if there is any statistical validity here. No wonder e.g. Yahoo failed badly at predicting the preelections for South Carolina: http://www.technologyreview.com/web/39487/
People are way to proud and keen on just being at all able to process the amount of data, they completely seem to neglect properly validating their performance.
Sorry to be this pessimistic about the state of the art.