Blizzard changed a lot of the specialties during Burning Crusade so you could easily switch to a different one by speaking to the appropriate trainer. In the case of Weaponsmithing, it appears that they retained the dungeon quests and unhooked them from the specialization process. Now any Blacksmith can do any and all of the Weaponsmith quests available in Winterspring. I was there on a farming mission recently and noticed that the quests were still showing up for me. I decided pick them up and try them out when I had a chance.
So earlier today I ran Sweet Serenity and Snakestone of the Shadow Huntress. I had already completed Corruption for the Swordsmithing specialty back in the day. I was curious if I'd be able to do the other two and get the recipes they reward, since originally you could only complete one of the three lines. The other two quests were locked once you turned the first in.
First I went to Live Strat and killed the Crimson Hammersmith. I might have killed a few other things while I was there.... oops! When you get to his area, you need to loot some plans on the ground to make him spawn. I got a copy of the Serenity schematic from those plans, unfortunately I already knew that particular recipe. Luckily they are BoE so I'm adding them to my Cataclysm auction stash.
Next I went to Lower Blackrock Spire (LBRS) and proceeded to get lost repeatedly. That map on WoWWiki sucks booty, you can't read the friggin' thing without an interpreter. Eventually I found the right area and killed the Shadow Hunter. I tried to find the end boss and get my achievement, but after another half hour of bumbling around I gave up and hearthed out.
Now I had two completed quests and various items in my bag to prove I was there and I did something commendable. Back at Everlook, I handed in my first quest and got the first set of plans. Then I handed in the next quest and got the second set of plans. It looked like the third guy was actually open to the idea of me doing his quest again, but I already had his recipe so I decided to call it good.
For those of you who are interested in collecting more old-world recipes, I recommend visiting Winterspring and picking up the old Weaponsmithing quests. Not only can you get the ones that were locked previously, but you might also get a new plan from the loot spawn in Stratholme. It's also possible that some of the armorsmithing quests are available now, I may give that a go in the near future if I get bored :)