Leeeeeeeeeeeeeroy!

Ok this is a long guide on how to get this from the VERY start [from my personal experience as a rogue].

Contents
-finding blackrock spire
-getting into UPPER blackrock spire, where the whelps are
-doing the achievement

Finding Blackrock Spire
Blackrock spire is where blackrock depths is: the place between searing gorge and burning steppes. Go there first. From the entrance to the mountain from searing gorge, turn left and find the chain that heads down to blackrock depths. Go up the chains, and turn LEFT (away from brd). For some computers, it might be easier to see if you raise your gamma, which can be found in the video options. Turn left and get off the chain, go past the hand that’s holding the chain, and go straight until you can jump onto a little balcony. You should see the portal into blackrock spire.

Getting into UBRS

note: as of 3.0.8, the door is now opened, so doing everything below is totally optional. you also no longer need 3 people, so its also very soloable.

When you get into brs, around 20 yards ahead there’s going to be a small bridge. On the ends of the bridge, the left end is the entrance to upper blackrock spire, and to the right is the entrance to lower brs. In order to open the door to ubrs, you need to get Seal of Ascension, which involves a quest chain. The door is not targetable (no gear icon when scrolling over), which means lockpicking or seaforium charges wont work.
The chain starts when you get an item called Unadorned Seal of Ascension, which is a random drop from any of the mobs in lower blackrock spire. You must get this item before you begin, so start grinding =). It dropped for me after around 25 minutes of killing as a rogue…for some of you it will drop within 2 mobs for others maybe even longer.
Once you get the seal, you have to go to an npc named Vaelan…theres a good video on how to find him posted by Lieandra on Vaelan’s page.
If you have the unadorned seal in your inventory, he will give you a quest which involves killing the 1st, 3rd, and last boss of lower brs.
Although LBRS may seem like a maze, just explore wherever you can explore WITHOUT FALLING OFF RANDOM CLIFFS (like i did), and you can eventually find all the bosses. For players who cannot stealth or aren't above level 65, getting a group is the best way to do this. For level 80's, this should be easy.
Anyways so after getting each of the above mentioned bosses' drop, return to Vaelan and he will give you a second quest.

For the second quest, you need to almost kill a mob in dustwallow marsh named Emberstrife who is about as hard as the the last boss of lbrs (who you should have killed anyways). He is in dustwallow marsh, south of wyrmbog inside a cave thats visible on the map.
Go there, and inside the cave you will find nothing but Emberstrife. In your inventory you should find Vaelan's Gift. There are 2 items inside: One that releases the Unforged Seal and one that mind controls Emberstrife. Before/while you engage him, use the unforged seal and it should release a small item labeled unforged seal of ascension. Whether you are soloing him or not, DO NOT KILL HIM. Burn him down to around 10% and use the mind control item. While controlling Emberstrife, use Flames of the Black Flight (not flame breath) towards the unforged seal. Although the quest log says your mind control time is very limited, its around 10 seconds, which is more than enough. This should all be very easy if you are level 80...the only thing you have to be careful with is not to kill Emberstrife, meaning don't overload on dot's and be careful with auto attack for melee.

After this, your unforged seal should read forged seal. Release the mc and kill emberstrife. Head back to Vaelan.

Once at Vaelan, hand in the quest and he should award you with the Seal of Ascension.

Now go to the door to UBRS with the ring in your inventory and the closed door should open.

Doing the achievement
Sorry for the wall of text above, tried to be as clear as possible. When you go in, you should see small rooms with orcs inside. At the end of the large room, there is a closed door. In order to open it, you must kill ALL mobs inside EVERY SINGLE ROOM. Once done, the door will open.
In the next room, you will see Pyroguard Emberseer with some non elites casting at him. At this point, you will need THREE PEOPLE one person (as of 3.0.8). To the left, there is an altar...you must click and cast, and soon the non elites will aggro. Kill them, and after using the altar again (not sure if using it is again is required), Pyroguard should be killable. Kill him, and the door to the rookery whelps should be open. Inside that room you will see a few elite dragons and around a hundred rookery eggs. Have someone pull the elites out and kill them...its ok to aggro a few eggs but just try to aggro as few as possible.
How the eggs work is this: once you get very near them, even while stealthed, they will hatch and spawn a rookery whelp, which dies after 2 minutes. The whelps that spawn will aggro if you dont walk away fast enough. If you are stealthed, you wont aggro the whelps. There should be a huge number of eggs here.

Sorry for wall of text, but there didnt seem to be any full guides so I just wrote one myself. Hope this helps =), and gluck all of you with your titles.

at level 70, it is soloable but much easier to bring a group like I did.
at level 80, this should be easy. its even soloable as of 3.0.8 since there is no longer a key and only one person is needed to activate pyroguard.

Roguemandu (jenkins and explorer and champion of frozen wastes lol) [Level 80, Korean server Zul'jin]

edit: thanks putts for the correction and updated this for wrath.
edit: more wrath edits...thanks putts and daggertooth

For historical accuracy, the criteria for this achievement should be to aggro 50 Rookery whelps in 15 seconds.

http://i5.photobucket.com/albums/y196/Bloodysaber/Jenkins3.jpg
Very, very easy to do with a few people. Get a little AoE, clear to the rookery.

Clear the MANY WHELPS and succeed where Leeroy failed.

You have a 32.33, repeating of course, percent chance of completing this achievement.