The secret wasn't a recipe:
Here is Notch talking about it on reddit:
We've tried adding secrets to the game before. Small things, like obscure crafting recipes or weird behavior, and everything always gets figured out immediately. No matter how obscure we make a new feature, it's fully documented within hours of a new release. This is awesome, and a great example of how dedicated some Minecraft players are, but it also means we can't really hide anything good in the game even if we tried.
So a while ago, I did some intentionally obscure code in the title screen to switch two letters around, making it say "Minceraft" (old running gag, there's even a "minceraft" mockup t shirt design we did) instead of "Minecraft" on every 10000th game launch or so, and nobody found it! I was so happy about that, I finally knew something about the game the players didn't know.
Flash forward to this GDC a few days ago, I'm doing an interview with Chris Hecker, and he asks me if there's anything nobody has found in the game, and I say yes. I should've said no, but I said yes. Then I start getting emails and tweets about it, people start getting excited, and knowing how minor the secret is, I try to tell people it's a very minor secret. That seems to fuel the flames. A reporter from a well known gaming site wants to run an article on it, and I tell him not to. Getting people hyped up about an intentional typo isn't really a good way to spend everyone's time.
There's a lot of cool stuff to learn from this, though. One is that it IS possible to hide stuff in plain sight, but once people go looking for it, they will find it. Another thing is that people seem to want to get excited over things, even if you tell them it's nothing major.
I'm impressed and relieved you found it. I won't comment on it outside of this subreddit.
Source: Reddit and Planet Minecraft