Tried summoning within Zul'Aman lately? If you have, you may have noticed that since yesterday's maintenance, it's a no go. Tigole confirms that summoning within Zul'Aman has been disabled due to an exploit (although we haven't heard what the exploit was-- it may have been something to do with the timer quests). It should be re-enabled soon.
And he says something else that summoners everywhere will be extremely happy to hear. Blizzard is planning to enable summoning from within instances to anywhere else in the world. No longer will you have to wait for that extra healer or that battleground PvPer to make it all the way into the instance before you can summon them and the get the raid started-- after this gets implemented, you'll be able to summon people to the instance (with a Warlock, of course), directly from anywhere else they are in Azeroth.
Huge change, and definitely will help not only raids to get started on time, but substitutions to join raids quickly as well. Not summoning in Zul'Aman now is a small price to pay for that update, which may come (Tigole says they're still working out the kinks) as soon as patch 2.4.