I don't know if this is something that's already been discussed to death elsewhere, but I've been wondering: since they're replacing Tennant what are they going to do about that whole ginormously important relationship that Ten!doctor is supposed to have with River Song sometime in his future?

IIRC, River never said she'd met this version of the Doctor before. She said that he was 'younger than she'd ever seen him', which didn't necessarily mean that she'd met say a 50year old looking Ten, just that whichever version 11, 12, 13 she'd met was older looking. So they could still have her meeting 11.