Honestly, continuity really isn't something one is supposed to worry about as far as the Bond films go. The series has been largely episodic, rather than serial, for most of its existence. Of course, we've had recurring characters and the odd reference to past films/events, which are supposed to indicate they all take place in the same broad 'universe', but I think that's really the extent to which one can rigidly apply logic to the continuity of the franchise.
Nevertheless, I've always considered the first 20 films to be in one continuity, more or less (with the caveat that with the Dalton and Brosnan films, a 'floating timeline' was applied to keep Bond relatively young). In terms of the broader narrative history of this Bond-he becomes a 00 agent at some unspecified point during the Cold War, takes down SPECTRE and Blofeld, is married and widowed, fights various other villains, and after the Cold War, works under a new female M. Precise dates and period settings don't really matter much 'officially' though everyone is entitled to their theories.
As far as the Craig films go, they're explicitly meant to be a reboot of the franchise, with a totally new continuity and timeline. Judi Dench's M in these films is a different character, continuity wise, from the M in the Brosnan films. Even if she's playing the 'same' character, its only the same character in the way Craig is playing the same character as the previous Bond actors. In this new timeline, Bond becomes a 00 in 2006, goes through the events of Casino Royale and Quantum of Solace shortly thereafter...and then we have a six year gap till the events of Skyfall, at which point he's a veteran agent.
When Mendes meant that he wanted the series to 'loop back', he was talking in terms of making a 'traditional' Bond film with an experienced Bond, something we arguably hadn't seen since Die Another Day. He wasn't talking about returning to the old continuity and timeline explicitly. At the same time, the new continuity may borrow aspects from the original, to use as homages, or simply to update them. Thus we have Q, Moneypenny, a male M and basically the set-up from the old Connery films. The Aston Martin DB5 is another such homage. All it really tells us, continuity wise, is that Bond at some point had the car weaponized. Whether or not Craig's Bond went through the events of Goldfinger is open to your interpretation. Likewise, whether or not Bond was being sent on the mission to investigate Dr. No at the end of the film, or whether Q's comment about exploding pens was a reference to the events of Goldeneye.
