I think every writer must go through a phase where they feel like a fraud. It might happen every once in while, or it might happen on a day to day basis. It definitely depends for me, I can tell you that.
Being published, being a bestseller, doesn't make the feeling go away. It can be just as strong with the evidence of your success (i.e. book covers) staring you in the face as it can be when you have none of that.
Some days I wonder when my editor will realize she made a mistake and I just don't have what it takes. Some days I stare at the story in progress and wonder how on earth I've made it through 7 books so far (7 that Harlequin have bought, not counting the under-the-bed books). I wonder when it's finally going to happen and the page will just stay blank because I can't possibly think of another word to put on it.
If you've ever felt any of this, believe me that you are not alone! It happens whether you've published several books or no books. I stumbled across a post by Bob Mayer that gave me several aha moments, and I wanted to share it with you. Go forth and read Bob's words of wisdom. Know you are not alone. :)