I just read the New York Times article, Probiotics: Looking Underneath the Yogurt Label.
My favorite quote: '"To say a product contains Lactobacillus is like saying you're bringing George Clooney to a party. It may be the actor, or it may be an 85-year-old guy from Atlanta who just happens to be named George Clooney. With probiotics, there are strain-to-strain differences."'
I'm fond of yelling at the television when a "probiotic yogurt" commercial comes on. All properly made yogurt (and many other fermented products), contain probiotics. They're the bacteria that create yogurt out of mik. Another thing to keep in mind, however, is the medium. An organic, naturally made, whole-fat yogurt is going to be better for you than a highly-processed, chemical-filled, de-fatted one like those normally advertised, which are harder for your body to digest.
For those lactose-intolerant among us, miso is another great way to get probiotics from food. Again, the kind that's made the old-fashioned way (like they've done in Japan for hundreds of years - or however long miso has been around) will be better for you than the kind that's been sitting in a hot pot all day at the local sushi place.
But I digress. Read the article. It's a good one.