At the beginning of each upkeep, creatures you control gain flying until end of turn if a creature you control has flying. The same is true for fear, first strike, double strike, landwalk, protection, trample, and vigilance.
10/1/2005 For example, a player controls three creatures when Concerted Effort’s ability resolves: one creature with flying and protection from red, one with islandwalk and protection from green; and one with vigilance. All three creatures will have flying, islandwalk, vigilance, protection from red, and protection from green until the end of the turn.
10/1/2005 Creatures keep all abilities granted this way until the end of the turn, even if Concerted Effort or the original creature with that ability leaves the battlefield.
5/1/2006 In Two-Headed Giant, triggers only once per upkeep, not once for each player.