Those who've tried to summon somebody inside Zul'Aman recently have probably noticed that the entire instance has been flagged as "no-summoning", similar to the entire zone of Netherstorm. It seems that there's some sort of wall-jumping exploit that allows players to defeat the chest timer quicker than intended, so:
We had to disable summoning yesterday via emergency hotfix due to some exploits. We're hoping to re-enable summoning as soon as possible. We know it's a big inconvenience and we apologize.
On a related note, we're looking into fixing some issues with summoning that would once again enable us to allow warlocks to summon people into an instance from anywhere in the world. This could happen as soon as the next big content patch (2.4.0) if all of the technical issues can be resolved by then.
It seems everytime Blizzard closes a door they open a portal to the twisting nether. The long-awaited return of into-instance summoning will be a great boon for many guilds who already have a hard time getting people to show up on time for raids.