The fantastic allegory of the 3 different views of life and their unification. The vinegar vat symbolizing the essential ONE truth and the three tasters with their three different taste buds symbolizing the different perspectives of the ONE single truth. 
To Confucius, the left most, life seemed rather sour. The present was out of step with the past and things were falling into disarray. A strict order was needed.
To Buddha, the one in the middle, life was bitter. All attachments and desires (symbolized by the vinegar) turned bitter in the end, and led to suffering.
To Lao Tzu (or Lao-Tse), the right most, the founder of the Taoism, life is sweet. Life is sweet, once one knows the natural balance of things. The world is governed by balance, and out of balance causes sourness or bitterness. It is the knowledge of being out of balance that leads us back to the eternal balance itself. The sourness or bitterness come from unappreciative and shallow thought process of the interfering mind. The conscious understanding mind will identify the bitterness and move towards the sweetness. Once understood for what it is, the balance of life (or the vinegar), is sweet.
frosted+glass 
Exactly.
Oh, you forgot to mention that you can actually get a poster or painting of the vinegar tasters from a master painter in china:
http://www.edepot.com/taoart.html
An alternative interpretation is that Lao-Tse, or the Taoist, doesn’t think the vinegar is “sweet”, but that it is good vinegar.
Good vinegar should not be sweet, it should be bitter or sour. In showing their displeasure, the Confucianist and the Buddhist have shown their lack of understanding of the nature of what they are tasting. Only the Taoist has appreciated the vinegar for what it is, hence is smiling.