Is it true that it's amateurish to start a book with a line of dialogue? What's your opinion?
Depends on what that line of dialog is and where you go after that.
Starting a book with dialogue is difficult. On the upside, it puts you immediately into the middle of something live. On the downside, you have to then work a little harder to establish your scene, sense of place, character, etc.
Starting with dialogue is not really a right or a wrong. I've seen it done well (Ender's Game & others by Card) and I've seen it done poorly (too many to mention).
Amateurish is not so much what you do, but how you do it, like using something cliché or cheesy. (Although, Kerry Blair did it in Ghost of a Chance and I thought it was funny.) Or something that is an obvious fake-out just to grab your attention, but then doesn't follow through.
After writing this, I googled to see if I could find some examples for you and lookie here—Nathan Bransford agrees with me.
Unfortunately, I couldn't find a list of books that start with dialogue. If you know of some that do it well, list them in your comments.