Text (SM): Players can't gain life. ; Damage can't be prevented. ; All damage is dealt as though its source had wither.
Spells and abilities that would cause a player to gain life can still be played and still resolve, but the life gaining part of them has no effect. [Shadowmoor FAQ 2008/04/21]
A cost that includes life gain cannot be paid. [Shadowmoor FAQ 2008/04/21] For example, Invigorate.
Effects that would replace gaining life with some other effect won't be able to do anything because it's impossible for players to gain life. [Shadowmoor FAQ 2008/04/21]
An effect that replaces an event with gaining life will replace the event with nothing. [Shadowmoor FAQ 2008/04/21] For example, Words of Worship.
If an effect sets your life total to a certain number, and that number is higher than your current life total, that effect will do nothing because it would be making you gain life. If your life total was lower, you still would lose life. [Shadowmoor FAQ 2008/04/21]
Spells and abilities that would prevent damage can still be played and still resolve, but the damage prevention portion of them does nothing. [Shadowmoor FAQ 2008/04/21]
Damage can still be redirected or replaced. It just can't be prevented. [Shadowmoor FAQ 2008/04/21]
The last ability works on damage from all sources (creatures, spells, and even cards not in play). Wither is applied in all cases. [Shadowmoor FAQ 2008/04/21]
