This is a question of level of language. In formal English, the correct question is "Whom are you talking to?" or in very formal English, some die-hard grammarians still insist on not placing the preposition at the end of the sentence, so "To whom are you talking?" (Placing the preposition at the end is called "preposition stranding" and all Germanic languages do this.)
However, almost everyone *says* "Who are you talking to?" and it is by far the most common way of asking that question.
Therefore, my advice would be to stick with "Who are you talking to?" unless you're in a very formal situation (on stage, speaking to a large group of people, before a judge in a courtroom, etc.).
When writing, you can possibly use "whom" a bit more, even though in modern day English, it has largely fallen out of favor. I teach ESL in school (English as a second language) and tell my students not to bother with "whom", just use "who". For those students that are interested, I explain the difference -- "who" is used to ask about (or to refer to as a relative pronoun) the subject of the sentence: "The man sees the girl. Who sees the girl?" While "whom" is used to ask about (or to refer to as a relative pronoun) the object of the sentence. "The man sees the girl. Whom does the man see?" So if you're going to use "whom", just make sure you use it correctly! :-)