I think Blizzard does this on purpose. "This" being that they give us a set time frame for the servers to be brought back online, and then announce a delay only 30 minutes before the scheduled go time. I call shenanigans! If it were an honest mistake, you'd think the company would have learned by now to stop giving a schedule for servers to be brought back online. But these delays keep happening, and have occurred constantly and consistently for as long as I can remember. Why on earth wouldn't Bliz take a different approach to their announcements that would help remedy the situation? Because they want you checking, rechecking, and re-rechecking for the servers to be back online, to build an uber reinforcing response, that's why.
There are undoubtedly some smart behaviorists over at Blizzard Entertainment. They know how to push buttons, both literally and figuratively, to get you to come back for more. Whether it be checking for the servers to be online, or trying week after week after week to get a specific item to drop off a boss in Karazhan, they have you exactly where they want you - addicted to WoW.
In general, addictive behaviors all pretty much work in the same way. You have a behavior that gets paired with a desired stimulus, and the response is that the behavior is more likely to occur. It's basic learning principles; reward, punishment, etc. They key factor in getting someone to perform a behavior over and over again, however, isn't just in the reward they get, but WHEN the reward is given. And when that reward is variable in nature (as in, it doesn't occur all the time and it's presentation cannot be predicted), it elicits the greatest behavioral response.
Imagine a gambler sitting in front of a slot machine. He feeds it with bills and coins and pulls the lever mechanically, not knowing when or even if it will give anything in return. Yet, there he remains. Why? The knowledge that the stimulus he's looking for, as unpredictable as it is, CAN happen. That unknown variable servers almost as a type of motivating factor, keeping the gambler hooked until it does happen (even though it never may).
Blizzard implements the same type of reinforcement schedule in WoW. Drops in instances or off any mob for that matter, the magnitude of stimulant variety that can occur, and even server schedules - they all reinforce our almost ritualistic, compulsive behavior to check and recheck, try and retry. It keeps us hooked, and Blizzard knows this.
Makes ya see server delays in a different light, doesn't it.