It isn't a prequel, in that is not set prior to Dr.No and all the other movies. It's a total reboot of the series, in the same way that Batman Begins showed you the origin of the character and had no relation to the previous 4 Batman films. I think I can guarantee that they will not remake any of the classic Bond films - They would be shot down in flames if they even suggested it. And the producers have said that they plan to continue the story in Casino Royale in the future movies.
So its best to think of this film as, essentially, an entirely new series, and not a continuation of any of the previous movies, or a prequel.