Okay, so I have several single-serving size containers of plain yoghurt, and yoghurt and fruit, in the refrigerator, with March code dates.
Usually, I would toss them, but I needed some plain yoghurt for a tandoori marinade. You know what, it tasted fine. No problems. Am I living dangerously? Apparently not.
From the wikipedia entry on yogurt:
Dairy yogurt is produced using a culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus bacteria. In addition, other lactobacilli and bifidobacteria are also sometimes added during or after culturing yogurt.
The milk is first heated to about 80 °C (176 °F) to kill any undesirable bacteria and to denature the milk proteins so that they set together rather than form curds. The milk is then cooled to about 45 °C (112 °F). The bacteria culture is added, and the temperature is maintained for 4 to 7 hours to allow fermentation.
In essence, yoghurt is milk that had already "gone bad" - in that it is fermented and curdled.
Many sources say that yoghurt should be okay for at least 7-10 days after the "sell date" if refrigerated. Sour cream, for instance, has a refirgerator life of 3 weeks after the "sell by" date.
By the way, the orignal yoghurts were made with camel and horse milk.