I don't see how it could have been her last kiss. This implies that the Doctor in the future gets a new haircut, takes River on a lovely picnic, gives her a sonic screw driver knowing she's heading for the Library, but doesn't kiss her? Ridiculous!
How do we know he hasn't already gave her the screwdriver in his timeline. Was River's sonic in Library different from Ten's?
I guess because he clearly hasn't given it to her in her timeline. The library 2 parter made it clear that the last time she saw him was for the picnic when he gave her the screwdriver, and given it was the first time he'd kissed her, it seems unlikely that it was late enough in his timeline for him to give her thescrewdriver, although I guess there's no reason why it couldn't be, except that she clearly gives the impression that the Doctor who gives her the screwdriver is much older, which would tie into the older 11 doing it (or maybe even 12 or 13)
I think unfortunately once we see the whole picture it's going to become apparent that their timeline won't make as much sense as we first thought, I'm guessing Moffat will go for the poetic version rather than a logical version.