As part of my preparations for Patch 4.3 I've been collecting some classic recipes I don't already have. This includes the full set of Black Dragonscale gear from Leatherworking. Most of the recipes are drops, but one of them is purchased from the Thorium Brotherhood vendor and requires Exalted reputation. I thought I had the correct reputation on the appropriate characters, but it turns out I did not. So I set out to correct this problem and ran into a few roadbumps. After searching for current information that reflects the Cataclysm changes, I decided to run the full process myself to verify how it works.
If you already have some Thorium Brotherhood rep and are just looking to boost yourself a bit further, you can still give the standard turn-in items to Lokhtos Darkbargainer. He'll take core leather, lava/fiery cores, blood of the mountain and dark iron ore in exchange for rep (350 for 10x Dark Iron up to 2000 for Fiery/Lava Cores). From what I've seen, this part has not been changed other than the rep values are dramatically increased. Wowhead seems to be off on their rep numbers, Wowpedia looks fairly accurate.
On the other hand if you don't want to buy tons of these items or they just aren't in strong supply on your server, you have the option of turning in Dark Iron Residue to Master Smith Burninate at Thorium Point. Here's where things start to skew away from the options of old. Burninate will no longer take kingsblood, iron bars, heavy leather or incendosaur scales. He only accepts Dark Iron Residue in stacks of 4 or 100.
Also, he won't speak to you at all until you complete the quest Welcome to the Brotherhood. This quest is essentially the final quest in Searing Gorge. I checked Burninate multiple times before I turned in this particular quest and even with Revered reputation from all the other quests I did, he did not become available until this one was complete. This doesn't match what other sites are indicating, so don't be fooled. After the quest is done you can give residue to Burninate and he'll give you 1500 rep (without rep bonuses) for each 100 stack you give him.
Bear in mind that in order to unlock Burninate you'll be doing enough quests to reach Revered without turning in any items at all. Many of the quests in this zone give 1000 rep each. There are about 35 quests in all and you'll need to do 32-33 of them to hit Revered from Neutral. If you're goal is to unlock Burninate you'll need to do all 35 for the achievement (that's about when you get to the "welcome" quest). I used a leveling druid with no reputation whatsoever to test this process, so this information should be accurate for newer players.
If you're an older player who already has some Thorium Bros rep, you'll have to decide which way is the easiest to complete your faction grind. Many of the Molten Core goodies are cheap enough now that you could just spend the money to buy them all and turn them in at the Grim Guzzler. On the other hand, if you're still sitting at Friendly or Honored it may be easier to just spend an hour or two doing the Searing Gorge quests and get to Exalted that way. It may depend on how much rep you need and what's available for purchase at the Auction House.