another rule question

marek14 (or anyone else), cards like dark sphere or nova pentacle... must they be activated only when it is certain that its target will become a damage source? or can you just simply pick a target and have the effect apply the next time it would do damage?
in other words, obviously, if someone casts, say a fireball, you could activate nova pentacle (assuming there is a creature in play), but could you also activate it and choose something like a prodigal sorcerer as the damage source even if the sorcerer's ability is not currently being used and the pentacle's effect will occur if and when the sorcerer's ability is used during the current turn?
i know that in general, that would not be a very strategic use for it, but i'd still like to know. cards that say "the next time a source of your choice would deal damage to you..." always made me wonder.
i guess a succinct way to ask would be, must there be a damage dealing effect on the stack for cards like dark sphere or nova pentacle to be activated?
in other words, obviously, if someone casts, say a fireball, you could activate nova pentacle (assuming there is a creature in play), but could you also activate it and choose something like a prodigal sorcerer as the damage source even if the sorcerer's ability is not currently being used and the pentacle's effect will occur if and when the sorcerer's ability is used during the current turn?
i know that in general, that would not be a very strategic use for it, but i'd still like to know. cards that say "the next time a source of your choice would deal damage to you..." always made me wonder.
i guess a succinct way to ask would be, must there be a damage dealing effect on the stack for cards like dark sphere or nova pentacle to be activated?