I would say (I may just be repeating what BameO has said) that, when it's a conversation, you can use either. But if it's one person talking and the other person/people listening (such as if someone is giving a lecture) you have to use "talk to". By the way, in case you're interested - there is another one, "talk at", which is slightly humorous and implies someone talking to someone else with great energy, but without any interest in their response: She talked at me for three minutes straight about the plight of the Amazon rainforest, and then ran away to talk at someone else before I could answer. I wouldn't use that in formal writing though.

Okay, i guess talk to someone emphasize the course of the speaker telling something to the addressee. And the other onw emphasize the interact of each other. I think the key point is the differences between the perpositions. Hope i can help u sincerely.