With Judi Dench confirmed reprising her role as M, it seems unlikely the film will take place before "Dr.No" but instead will be a sequel to "Die Another Day" and the other nineteen Bond films.
It's made quite clear by dialogue in "Goldeneye" that Dench's M is a new M....so if she's in it, this film would presumabely take place during or after the Brosnan era.
However, the Bond series has always had a loose continuity of sorts ( There have after all been six Bonds, and, Moore's last few films aside, he hasn't really aged since the 60s!) .
Although I suppose one could argue that Judi might play "M" before she became M...but she's noticeably older than she was in "Goldeneye" so that might not work out.
BTW apparentally the film will start filming in January, although a Bond girl and villain, as well as a new Moneypenny, have yet to be cast.