When should you side lands in and out?
A recent case of this was in pre-Avacyn Restored Standard. There were some versions of UB Control that had 3 Nephalia Drownyards main-deck and another one in the sideboard. 

In a control mirror with both decks full of reactive cards, the player who has mana advantage usually wins the game, because he or she can play many spells in one turn (e.g. play a threat, counter a removal spell against it, then counter the opponent's counter).

With lands in the sideboard, this deck could side in an extra land, taking out a removal spell. This way they would hit their land drops more easily and have another win condition in the deck, since milling was the best win condition in UB mirrors.

For reference, the 2nd place finisher of GP Baltimore 2012, as Hackworth pointed in the comments, used 2 lands in his sideboard. Its player goes so far as to say that Nephalia Drownyard is "the only card that matters in the mirror."