You'd have a better chance of stealing the crown jewels than you world stealing a copy of the book. I've seen a list of every last security precaution they're taking, so believe me, it's bloody difficult. Satellite tracking, 24-hour security, threats of legal action and so on and so forth. Last time the leaks only occured in the day or two before the release; Raiks' infamous post of the page where Snape kills Dumbledore was only posted the night before the launch. I remember a story about a girl who actually wrote her own version of Order of the Phoenix because she couldn't wait for it to come out. While they were nowhere near alike, her parents paid for a single copy of the book to be printed and bound as if it were the actual book. It wouldn't be difficult for someone to do a forgery and pass it off as the real thing, especially since only a handful of people know what happens.
Now for what I really want to talk about with this post (Malfoy's post distracted me), and that's another wild RAB theory:
What if RAB is indeed Regulus Black, but not Sirius' younger brother and not someone using his identity? What if RAB is actually Sirius' father and that Sirius' brother was named for him? It seems to be fairly common in the wizarding world; Harry's middle name is James, Barty Crouch Jr. was named for his father, as was Tom Marvolo Riddle, who wasn't just named for his father, but his grandfather as well. I wouldn't be surprised if it's far more prevalent than this, but we haven't met anyone else named for their parents or a relative. Now, before you start saying that Harry, Barty Jr. and Voldemort are only children and Regulus is Sirius' younger brother, take a look at Ron. He's also named for a relative - his great uncle Bilius according to JKR - and he's certainly not the oldest Weasley. There's four examples of four major characters (even if Barty Crouch was only in one book) all of whom have been named, in some way, for their parents. The Leaky Cauldron - or someone similar - "leaked" RAB as standing for "Regulus Arcturus Black" a while ago, but if you really think about it, we don't actually know the middle name of Sirius' brother. "Arcturus" might be the middle name of Sirius' father, and following the Black family traition of naming children after stars, there's plenty of stars that being with an "A": Aldebaran, Antares, Arneb, Altair, Adara, Acrux, Alcaid, Alhena, Alphard, Algol, Alpheratz, Alspidiske, Almach, Algeiba, Ankaa, Aldura, Avior and Alderamin. There's probably many more, but Wikipedia doesn't list them all. As you can see, there's plenty of possibilities. Now, we know next to nothing about Black Snr. (Sirius' father); only that he was "almost as bad" as his wife. We also have no idea as to when the horcrux was taken from the cave, nor do we know when Black Snr. died. I don't think it's outside the realms of possibility that, rather than having the younger Regulus survive the Death Eaters or escape them, that Black Snr. heard about his son's death and devoted himself to bringing about an end to Voldemort's reign because his son died. Black Snr. discovered the cave and somehow got the horcrux (there's a fan theory that Kreacher was involved and his current condition was caused by drinking the potion) and left a fake behind with a message for Voldemort. He then kept the genuine horcrux either because he didn't know what it was, didn't know how to destroy it, or was well aware of it and intended to use it as leverage over Voldemort (highly unlikely, but he was a Slytherin). Now, JKR has said that fan guesses about RAB being Regulus are "a fine guess", and the name often translates into whatever the local language is (it's "RAZ" in Holland). People have immediately jumped to the conclusion that RAB is Regulus, and therefore Sirius' younger brother who somehow miraculously survived. What if, in being right about the identity of RAB, we're completely wrong? What if "Regulus Arcturus Black" is not Sirius' younger brother, but in fact his father?