Apparently a handful of the "lost" recipes in Cataclysm were put back into the game as undocumented changes for the 4.1 patch. Yesterday I stumbled across the Shadow Oil recipe in the process of training my Junior Alchemist. After that I hit up all the various trainers and found quite a few of our missing recipes. I've written about this previously and some of those items have reappeared. For the record, none of these changes were mentioned in the patch notes.
Low level Engineers are getting one recipe back - the Lovingly Crafted Boomstick. Sadly we are still missing the Ice Deflector and Deepdive Helmet recipes. Personally I don't expect the Ice Deflector to be revived since it's a limited use item, arguably the Deepdive Helmet is as well. If you want to pick up the Lovingly Crafted Boomstick recipe you should visit your nearest Engineering trainer.
Unfortunately none of the missing Leatherworking or Blacksmithing recipes were resurrected in this patch. I know Leatherworkers in particular are lamenting the demise of the Deviate Scale recipes, they were strong items for low-level leather wearers. Blacksmithing was only missing one recipe for the Solid Iron Maul. I don't think this was a particularly valuable item so it may not make a comeback.
Cooking got the major love in 4.1 with three lost recipes making their way back into the game. Two of the recipes in question were key to Cooking progression in the 250-300 range for players who don't fish. Charred Bear Kabobs and Juicy Bear Burgers were only missing on Horde vendors, but they were sorely missed. I'm not certain why Fillet of Frenzy was revived since there are plenty of other recipes in the same range, both fishy and non-fishy. In any case, all three of these are now available from the Cooking trainers.
Feel free to report any other "MIA" recipes you happen to stumble across in Azeroth here for the benefit of other players! If you're not sure what those are, skim through my older post: Cataclysm Vendor Changes.