Is this common knowledge? I'd never heard this before. (I'm not complaining, though, because I got my book a week earlier than I planned.)
I don't think it's commonly known, although it's not really a secret. I wish I had a picture of our receiving department, because it's really not as big as people think. There's no way it could hold each book that arrives until the anticipated sale date. Basically, what you see on the shelves on the book floor is what we have, unless something just arrived in the day's shipments. So, only a selected group of books are SOS (determined by the publisher) and they come in boxes clearly warning not to break the date. Those are the only books where the release date matters. Otherwise, books are on the shelves within a day of when they're received, regardless of the projected release date.
An example of the audio exception is happening right now with the new book by Sue Monk Kidd (The Secret Life of Bees), The Mermaid Chair. We can't sell you the book until April 5 because of SOS, but in this case, the audio isn't SOS (sometimes it is), so you can walk in and buy the audio format today.