This is a potion that is only availible to monsters and will come out in 1.4 PC edition.
Wither is a potion delivered through contact of Withering mobs such as the Wither or the Wither Skeleton. It tints the hearts of your charecter black and you take damage. The potion is only availible to players if they hack it in or they get it due to mobs hurting them. If the player dies of this effect, the death message will say...
<player name> withered away.
It is best to avoid the wither mobs at all costs unless you want to fight the Wither itself.