In most contexts, there is no difference. "Lets talk [x]" is an informal, colloquial abbreviation for "let's talk about [x]".
There is something of a difference though - the shorter "lets talk [x]" can imply things that the other does not. For example, "let's talk salary", following a discussion about potential employment, is really like saying "tell me what you are offering me as a salary". Talking "about" something is really just naming the subject, and is not always so pointed.
I would also note that I believe it is more common in American English than British English, although it is used.