I did a quick search, and have stolen the following quips from several websites...
"Even now, many people believe that if they eat yogurt they do not have to take acidophilus. I wish it were true. The commercial yogurt which sits on the grocery store shelves for days should not be considered as a source of acidophilus. First, yogurt is not usually a source of acidophilus. Secondly, when yogurt sits for days, it produces acids which kill bacteria. So the yogurt does not serve as a source of friendly bacteria. On the other hand, home-made yogurt is a good source of beneficial bacteria. However, the amount it provides does not normally serve the purpose. Therefore the need for acidophilus supplementation exists." (Good link below.)
"Since 70 to 80 percent of females and 30 to 40 percent of males have some degree of yeast infection, and many are allergic to milk and dairy products, it is a good idea to use non-dairy acidophilus."
"Yogurt must by law contain at least 10 million bacteria per gram at the time it is marketed."
"The National Yogurt Association (NYA) established its own criteria for live and active culture yogurt in conjunction with its Live & Active Culture seal program. In order for manufacturers to carry the seal, refrigerated yogurt products must contain at least 100 million cultures per gram at the time of manufacture." (1 gram = 0.03527 ounce, 1 oz
= 28.350 g)
"Kefir (similar to yogurt) will not 'go bad' in the sense of turning sour, but the bacteria count will go down significantly after about 45 days, so you should drink it within 6 or 7 weeks."
Check this site out: http://www.chiroweb.com/archives/09/18/21.html