I can confirm (I looted it yesterday) that Formula: Enchant Weapon - Crusader is back in the game as of patch 4.1. It drops off of Scarlet Archmages in the Tyr's Hand region of Eastern Plaguelands. These mobs are no longer there if you have done the Tyr's Hand parts of the Eastern Plaguelands questchain, which means you won't be able to farm this on your achievement toon. The drop rate is still pretty low, so be prepared to settle in for the long haul if you're farming this yourself.

People seem to be paying a slightly inflated price for the formula still. (Some people don't know it's back in the game, I guess.) So while I went out to farm this for myself, I ended up selling the formula instead of learning it.
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