Blizzard's Lead Encounter Designer Ion Hazzikostas, a.k.a. Watcher, has taken to the forums a few times lately, along with Community Manager Zarhym, to provide clarification on Challenge Modes.
With all the bad air swirling around Blizzard from certain minority groups within the WoW community about catering to casuals, challenge modes are something they seem very keen to stand firm on. When asked if challenge modes would ever be nerfed, Ion replied as follows:
Zarhym also weighed in with his own input on the nerf question.