For the answer to this question, we might turn to Martin Lindstrom's book, buyology, which argues that humans are rather poor reporters of our own beliefs. Whether or not you agree with Lindstrom that in the future we'll be developing brands at the bedside of patients undergoing MRI scans, his argument that we can't rely on people's self-assessments is pretty convincing. In effect, he suggests that, in market-research situations, people say what they believe others want to hear, rather than what they actually think.
Twitter is a very public environment. Its users may be encouraged to say what they are thinking but I'd argue that, in reality, they'll be writing what they think others want to hear. Peer pressure and the desire to conform don't magically disappear online: if anything, the permanence of a person's web presence makes the need to say the 'right thing' even more pressing. In that respect, the idea that Twitter provides us with a vast database of consumer's ideas - the planner's ultimate resource - stands on shaky ground.
That's not to say we should dismiss the potential opportunities that Twitter provides us with. Responsive brands will take the service and run with it, exploiting its reach and appeal to their advantage. But I think that to see its introduction as a 'seismic point' in brand conception is misguided.