This is an interesting question- since all people have bugs in their GI tract, and probiotics are friendly bugs, then everyone should take probiotics (disclaimer: I'm the CEO of Dannon).

I would say that the only contraindications I could think of are allergies, and infection risk.  Obviously if you are allergic to the probiotic composition, you can't take it.  This leaves infection risk.  Some conditions (AIDS, chemotherapy, bacterial overgrowth) weaken the immune system so much, that theoretically, even friendly bugs may get into the blood and cause havoc.  So in these conditions, it may be risky to use them.  However, there is the argument that probiotics may still prevent an even worse GI infection in these cases.  So anyone with a history of a GI infection may benefit as well.

Well, in the end, I don't really know.  My guess is that almost anyone with chronic GI problems might benefit from probiotics. So everyone run out and buy some Dannon :)