Hmm....I kinda want it to be an actual recipe in the game for the Quiver or something. If it actually is a recipe, I'd like to see it as a Brewing Recipe for that Bottle o' Enchanting, like brewing an Instant Health II potion with a Diamond or something.
Like the previous poster said, there is no 'secret' item, as we know the entire list of in-game item codes. Honestly, I'm more willing to believe that Notch is just trolling us (think of Herobrine) than that there is some secret recipe no one's found yet.
He could have hidden one item or block from the terrain.png or item.png in a seperate file which was converted to code, which is why there isnt a png for it.
On a thread started about this secret, this server was volunteered for testing purposes. (deuce.minexen.com) Soon after, someone placed a sign and we captured this footage. This can be verified by several others, any of whom were on that server last night.