Q: I have heard that it isn't really necessary to scald milk, even though the recipe may tell you to.  Is this right?

Milk is scalded by heating it to 180°F (82°C).  Visually, at sea level, this is the point at which tiny bubbles begin to form.  Because water boils at a lower temperature at higher altitudes, this visual clue may be inaccurate (see High Altitude Cooking).

Scalding serves three purposes: it kills harmful bacteria that may spoil the food being prepared, it destroys enzymes that may affect the way the milk performs in the recipe, and it raises the temperature of the milk to speed up results.  With modern pasteurization, the bacteria and enzymes are already destroyed, so scalding is no longer necessary to accomplish those goals, although heating the milk may help to encourage the growth of yeast in breads, to better dissolve other ingredients, or to promote desirable bacteria growth for recipes such as making yogurt.

In the case of raising the temperature to speed results, the milk only needs to be heated to the optimal temperature, not necessary all the way to a scald.  Temperatures might be in the range of 110°F (43°C) for making bread or yogurt.  Always check the recipe to be sure.

The one exception to note is that, according to Shirley O. Corriher in her book Cookwise: The Hows and Whys of Successful Cooking,  there may be some evidence that certain proteins in milk may affect the rise of breads.  For this reason, she continues to scald milk used in bread baking, as a precaution.

Of course, if you are using raw, unpasteurized milk, then you need to  scald it since the bacteria and enzymes have not been destroyed through pasteurization.

Addendum:  Thanks to Alton Brown in his recently aired Good Eats episode "Churn Baby Churn 2" for reminding me that there is a fourth purpose in scalding milk, that being that heat increases the amount of flavor that is extracted from some ingredients, such as vanilla beans, for those recipes where other ingredients may be added to the milk while it is being heated.