Basically "you are asked" is present tense and "you have been asked" is past tense.
If someone says: "You are asked...", it's usually a question asked by someone who is just repeating what someone else said. For example:
"You are asked (by person A) to write a letter. What did you write?", asks person B.
"You have been asked" will be used to form a tag sentence: "You have been asked to write a letter before, haven't you?"
Hope this makes sense ;)