Well, "talk with" suggests an exchange. "Talk to" suggests speech going from one person to another. Really, it's the difference between "with" and "to". When they apply to "talk", it's a really subtle difference, and you won't be wrong if you use either "talk to" or "talk with".

There is effectively no difference. I prefer "talk to".

I agree with Peachey. Casually both can have the same meaning, but "talk to" can cause misunderstandings. Example: "I need to talk to you." This could mean: I have something important to tell you. (You listen; talk.") OR it could mean that you have a complaint about that person that you are preparing to explain. In these cases it is not seen as an exchange. I will talk; you will listen. "It was nice talking with you." "It was nice talking to you." For most people these mean the same thing. But I will confess, when I hear the first one "talking with you" I get a fuzzy warm feeling because I do notice and feel the difference. Most of my friends know this about me too. :P But I think I might be a bit picky about language. ("A bit?!" my friends are probably screaming.) :P