I wouldn't say he's a sure thing yet. The next Bond film might not even go into production this year, and Mendes has been known to walk from projects in the past. It wouldn't surprise me if he turned out to be another Roger Michell, if the situation with MGM drags on for many more months.
A few points (which no doubt Zorin and others will say is totally wrong but meh)
They learned their mistake with Mitchell and aren't repeating it with Mendes right mow Mendes is currently adding his thoughts to the script. Basically the script is going back and forth between the various parties (director Producer Writers) compromising on certain aspects. Nothing major mind you just trying to match the Directors Vision of bond 23 with the Producers With the Writers all 3 parties want to make an amazing bond film.
the second thing I want to talk about is the Scott brothers their interest in MGM make me feel well quite comfortable with MGM,. Oh and expect an announcement that Sam Mendes is Directing bond 23 (and the release date) By June.
Of course now I'll let Zorin say i'm all wrong.