Blizzard has stated that Inferno bosses will have enrage timers, but that they won’t be unbeatable once these have expired. The idea behind the enrage timer is to get players thinking about creating balanced builds that will be able to handle Inferno bosses efficiently, rather than “playing it safe” with very defensive builds. Blizzard doesn’t want to encourage situations where players are chipping away at Inferno bosses for 30-45 minutes or more, just to cheat death. That sort of gameplay isn’t really all that fun on a regular basis.
The idea behind the enrage timer is to give players a sense of urgency – while not making the rest of the fight impossible. Players will want to down the boss(es) before the enrage timer hits – but it shouldn’t be seen as a deal breaker – unlike in Wold of WarCraft where enrage timers almost guarantee an immediate raid wipe.
Quote from Bashiok
I know what some boss enrage timers are, but to save spoilers I’ll transpose it onto the Skeleton King. So, as an imaginary example, in Inferno if you don’t kill the Skeleton King within four minutes he starts spawning three times the amount of skeletons. That creates a much tougher situation, but it’s one you could very well handle if played right.