In Burning Crusade, gaining 5% hit (Gear, Talents and Buffs) made you hit capped in heroics. (1% miss chance)
Similarly gaining 16% made you hit-capped in raids. (1% miss chance)

In WotLK, rather then being capped at 1% miss, we can raise our hit a further percentage, reducing our miss chance to 0%.
So heroics or 82nd level mobs will likely have a hit-cap of 6% and raids of 17%.