Casino Royale is definitely a reboot. The thing about the past Bond movies is that they never directly contradicted each other. Events in one movie were never wiped out by events in another. But with Casino Royale, there are several direct contradictions. Bond got his Aston Martin from Q in Goldfinger. In Casino Royale, he wins it in a poker game. Bond and Felix Leiter first met in Dr. No, then he lost a leg in Licence to Kill. In Casino Royale, Bond and Felix, both legs intact, meet for the first time. In GoldenEye, Judi Dench replaced Robert Brown, who had himself replaced Bernard Lee, who was first seen in Dr. No, when Bond was already an experienced 00-agent. In Casino Royale, he's just been promoted to 00-status, and M is played by Judi Dench, which is wrong, she should have been replaced, having made the role her own or not she should have been replaced. Finally, Casino Royale is very definitely set in the modern day. There's a direct reference to 9/11. Dr. No is very definitely set in the 60's. So no, Casino Royale is very definitely not a prequel, it is a reboot. It is Bond Begins