I just learned from looking at the official Diablo 3 site that there are a variety of Blacksmith Plans that you have to require in order to craft certain rare and legendary items. I have not seen any of these drop so far in my game, nor have I seen them for purchase at the Blacksmith or other vendors.
Does anyone know how you get access to these plans?
According to Blizzard, there are 3 Normal, 87 Rare, and 39 Legendary Blacksmith Plans. We know that the three normal ones are dropped by Izual in Act 4 on the respective difficulty levels.
Anecdotally by forum posts, it looks like people are finding one or two per difficulty (bnet forum post) from random monster/champion/boss drops, although one person notes none found on his way to Inferno. So far the only non-Blizzard list this gamer found was at d3db, and it just lists "Drop" as the source of each of the plans.
Any rare mobs can drop the pattern. I have a friend whose highest character is a Wizard in Act 3 Hell and he has already managed to obtain 3 legendary item patterns, compared to me who has 1 in Inferno Act 3 and Inferno Act 1 and haven't seen 1 drop as yet. The plans are just random but as others have mentioned I have found several rare item patterns in Inferno but typically the basic armour upgrades - nothing outstanding. The only way to obtain the plans is by persisting with the game and killing as many rare/elites are you can. Most people say they get them in Inferno because they farm Inferno for those 1k+ dps weapons or elite items. I don't recall any high level player going back to Normal/NM/Hell to farm when they can quite sufficiently farm Inferno.
I know that the merchant 'Squirt' in act II can sell blacksmith plans in the miscellaneous section. I dont know if you have to be at the end of act II or if you can periodically check throughout the Act, but I saw 5 rare plans, they cost 1.5 mil gold each, and that was on Normal Diff
I bought Diablo 3 for XBox, and so far I got most my drops for blacksmith on nightmare and inferno difficulty level, desert level, as well as final act at the spire and act 3 where you have to kill ballistas. Down the last bridge.