There's plenty of great coverage elsewhere of the details, and I just want to make a couple observations.
First, the UN's policy of moral compromise and appeasement towards Arab Muslim extremists has failed to protect them. It took a bombing to make the Saudis sit up and take notice of terrorism; they thought that if they supplied the fanatics with money and moral support that they could export the terror to other regions. After the Saudi bombing, however, the kingdom began a more serious crackdown on the Wahabi imams who were instigating much of it. It remains to be seen whether or not the UN will examine itself and ask "why do they hate us?" or if they will simply blame the US.
Unfortunately for the UN workers, UN operational policy is largely determined by France, Russia, and China, and until those countries get hit directly they may not have much motivation to change their positions merely because of attacks on UN personel.
2. It's strange that terrorists would attack the UN, considering that the UN doesn't really do much to interfere with their agenda. The UN was most likely attacked because it's high-profile, and it was a fairly soft target. The Coalition Provisional Authority headquarters are surrounded by military units and have secure perimeters that are virtually immune to this sort of truck bombing. Sure, a bomb could damage or destroy a gate and a few soldiers, but there's no way to get to the civilians inside.
Will the UN request coalition military protection? Will the UN ask to relocate to within the CPAs secure areas? Will the CPA grant such a request? I suspect the answer to all three is "yes".