An early (and yet quite complex) statement of the doctrine of sunyata can be found in the teachings of Nagarjuna found in the Madhyamika. Nagarjuna lived and taught in an era when a large number of Buddhist texts, both Mahayana and Hinayana, were becoming available to a wider ranger of students, as feats of writing and translation widened the reach of local teachers and ideas. Studying this wide hodge-podge of texts and teachings, Nagarjuna could not help but notice that a great many teachings attributed to the Buddha seemed to contradict one another. In the post-modern haze of the present century, scholars can point out that many of these texts were created years apart, often quite far from the homeland of the Buddha, and were attributed to the Blessed One out of a notion of humility we also find in a number of Greek schools of thought (and it certainly helped bolster the acceptance of these teachings). Nagarjuna had no such luxury; while he may have had some idea that he was dealing with works from multiple sources, it seems clear that he felt that the majority of the teachings he encountered, even contradictory ones, all fell from the mouth of the Buddha at some point.
Nagarjuna's means of reconciling these apparent contraditions was a true innovation. Nagarjuna pioneered the idea that Truth lay in delivery from error, rather than in any sort of positive statements about the 'external' world. As the mind was foremost in the perception and creation of the world, making 'true' statements about external phenomena was a useless exercise. Real truth was found in seperating the mind from deluding ideas. So if the Buddha spoke of the ubiquity of change to one student, and to another spoke of something as being eternal and changeless (as Nirvana was sometimes attributed), it was not that the Buddha perjured himself, or intentionally led astray his students. In the first case, he was dealing with a being attatched to and deluded by the notion of permanence. His response was to 'treat' this delusion by teaching change. In the second case, the being concerned had become attatched to the notion of change, to the point that it caused delusion and suffering. The remedy was to teach the presence of a permanent phenomena.
In this way, Nagarjuna and the Madhyamika school regarded the teachings of the Buddha as a sort of psychological 'antidote' or remedy, in the sense of Indian ayrvedic medicine- an idiom in which the Buddha himself sometimes taught. Delusion was the sickness that afflicted the mind; through his powers of perception, the Buddha would diagnose this affliction, and treat it through the application of teachings designed to seperate the patient from their conceptualizations. Truth was sunyata, emptiness and negation- the elimination of wrong ideas which were not meant to be replaced with 'right' ones, or with anything else for that matter. Remember- in Buddhism, there are no nouns, only verbs.
If you got all that (I didn't), please /msg the Dalai Lama. He'll be very happy to hear it. | 
