I'm kind of divided on this. On the one hand, I absolutely agree that the Skill Challenge must be dynamic, flexible, malleable; that the fiction (and attendant pressure) must morph to accurately represent the PCs' progress towards their goal. However, on the other hand, I've always tried to adhere to the advice given about SCs (and similar mechanics in other games) to make sure the terms of success and failure are clearly articulated and agreed upon before engaging the mechanics. It therefore becomes a very delicate balancing acts at times to adhere to both of these precepts.
Ultimately, I don't think it's a bad thing if during the course of a SC it becomes clear that the preset stakes and outcomes need to change to reflect the emergent fiction, so long as all parties agree to said change. Thoughts?