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.