It's nigh-ubiquitous in Wrath of the Lich King. You can pull it out of the ground pretty much anywhere. It's crafted into armor and weapons from powerful epics to crafted items to increase our skills. It makes up the walls of Icecrown Citadel, and the bones of Malykriss, the Lich King's planned replacement for Acherus. It makes up the bulk of the Scourge's material for its mindless warriors as well as its fortifications and siege engines, and the Vrykul even presses the living into slavery in Icecrown in order to meet their master's demand for more and more of it. But this is no ordinary metal: the Tuskarr call it the "Black Blood of Yogg-Saron" and whisper in hushed tones that it may yet shake the pillars of Heaven. (Well, I assume they do. Fella named Jack Burton told me that.)
Unlike cobalt and titanium, the metals that seems to occur naturally in Northrend, saronite's presence is due to the presence of a trapped old god. As you no doubt deduced from the name, Saronite is nothing less than a creation of the terrible Yogg-Saron himself. The enslaved miners toiling for the Vrykul near Ymirheim eventually go mad, and even if freed, hurl themselves into the depths to seek communion with the god of death. In his hubris, the Lich KIng seeks to prove his mastery of death, his transcendance of the state by exploiting the products of Yogg-Saron's imprisonment.
But as Yogg-Saron himself says, "No king rules forever." And as we storm the very gates of Icecrown Citadel and make our way through the Forge of Souls, past the Pit of Saron where even deeper veins of Yogg-Saron's Black Blood are unearthed, and stalk the Halls of Reflection themselves it seems clear that the dependence of the Scourge on the death metal may be its undoing... if it isn't ours first.
You first start running into Saronite early as you proceed through Northrend. In Whisper Gulch in the north of Howling Fjord you'll run into madmen who have fallen to the siren call of the old god via working with the tainted metal. Both the Alliance and the Horde have their difficulties with these lunatics, and both are understandably intrigued. While Yogg-Saron's influence pops up again in the Grizzly Hills, that doesn't seem directly related to saronite itself, but rather the roots of the failed world-tree Vordrassil penetrating into Yogg-Saron's prison. Still, the madness of the Grizzly Hills furbolg and the corruption of Ursoc bear striking similarities to saronite's effects.
In the Dragonblight both the Horde and Alliance run into saronite and its effects. Borus Ironbender sends Horde players to investigate the Scourge's weapons and to retrieve samples of the so-called 'Black Blood of Yogg-Saron' (it helps if you bring your Six-Demon Bag for this) while Alliance players discover their new saronite mine is overrun with the Scourge and confront the mystery of this strange bleeding ore. The corpse of Slinkin the Demo-Gnome also provides a journal detailing the strange, almost contemptuous manner in which those few Scourge who can think for themselves refer to Yogg-Saron even as they mine up the metal associated with him.
Saronite makes a return appearance in Zul'Drak as players raid a crashed Scourge necropolis in the quest Pure Evil, which gives players a chance to watch the luminaries of the Argent Crusade try their level best to defeat a small chunk of ore and fail spectacularly. The interesting thing is, by this point if you're a plate class you're probably wearing some saronite gear or at least have been working it for some time if you're a blacksmith. (Not even counting epics or the various saronite gear that starting Death Knights acquire, there's a lot of saronite gear in the game for leveling characters.) The discovery that you're wearing the hardened blood of an old god on your head or crotch or what have you has been detailed before (Disgraph focuses on a different material explanation for saronite so if you're offended by poop jokes, don't click that link.) but in all seriousness I have to believe personally that the entire explanation for saronite has to creep people out just a little.
There's tantalizing links between Icecrown and the Storm Peaks, especially Ulduar, that revolve around saronite. Darkspeaker R'khem in Icecrown's Saronite Mine is held captive by the Vrykul toiling away for the Scourge, and like General Vezax he appears to have saronite for blood: Vezax himself comes right out and says that the Black Blood of Yogg-Saron courses through him, giving rise to vaporized saronite that can even coaelsce and attack as an animus. This, combined with the madness that those exposed to saronite for long enough, brings questions: is the end result of saronite madness and corruption transformation into a faceless one? Are the insane miners in Ymirhelm the source of the faceless ones patrolling the old Nerubian city beneath Icecrown Citadel? Is Vezax himself a transformed entity, changed into a walking monstrosity by exposure to the elder god's blood, tainted and twisted by saronite?
Considering I'm carrying around two axes I got in the Pit of Saron, maybe I should be considering this more closely.
To some degree the Scourge seem immunized to this process of corruption due to their being dead already. Herald Volazj, who is at war with the Scourge in Ahn'Katet on behalf of his master Yogg-Saron, seems to make statements that indicate that the Scourge stand apart from both life and death. (His entire quote list may be found here.) While the quotation in question could refer to the old gods themselves, considering that Ahn'Katet is under attack by the Scourge the phrase "They do not die; they do not live. They are outside the cycle." at least could be aimed at them, and it could explain both why the Scourge can make such free use of saronite and why they prove so difficult for the servitors of the old gods to defeat. If they're immune to saronite corruption, one of the best weapons of Yogg-Saron's servants (turning your enemies into allies) is lost, while the Scourge themselves can perform that very same trick by killing your own minions then raising them as undead. Saronite corruption, a powerful weapon of subjugation, seems to have its teeth pulled when dealing with those who stand outside the very cycle Yogg-Saron exemplifies as a god of death.
As we march into Icecrown Citadel, saronite continues to make its presence felt. Despite Yogg-Saron's 'death' in Ulduar (raise your hands if you think he's really dead... yeah, me neither) the Scourge continues to mine the stuff out of the Pit of Saron at a prodigious rate. It's used as material in the reforging of Quel'Delar, which turns out to be a bad idea (hey, maybe we shouldn't reforge this sword out of the solidified essence of a god of pure evil... you think? Maybe?) and it's used in the creation of Shadowmourne by the Ashen Verdict, who claim to have finally figured out a way to bend saronite to their will instead of being bent by it. And of course, every step you take in Icecrown Citadel, every weapon or metal armor you loot from the fallen enemy, you're surrounded in the pulsating, breathing metal forged out of the black blood of death itself.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.