Anyone find one yet? Apparently if five of your Pokémon find Shiny Leaves, you get a leaf crown or something plus a star on your trainer card o_o!
From Serebii:
"The Shiny Leaf is discovered by talking to your partner Pokémon while in a bit of grass. However, the areas in which your Pokémon can find the Shiny Leaf are few and far between. The location depends solely upon your Pokémon's nature and it can be one of several areas so you'll need to keep checking your Pokémon. It's not guaranteed that you shall find it even if you're in the right area. If you have found it, your Pokémon will start with an Exclamation Mark, do a dance swaying from right to left and then have a green musical note over its head. Your character then immediatelly attaches the Shiny Leaf to the Pokémon. As you get more Shiny Leaves, the likelihood of your Pokémon finding one quickly decreases."
Link:
http://www.serebii.net/heartgoldsoulsilver/shinyleaf.shtml
So I guess this is just something else to do once you've done everything in the game, or if you're really bored o_0
From Serebii:
"The Shiny Leaf is discovered by talking to your partner Pokémon while in a bit of grass. However, the areas in which your Pokémon can find the Shiny Leaf are few and far between. The location depends solely upon your Pokémon's nature and it can be one of several areas so you'll need to keep checking your Pokémon. It's not guaranteed that you shall find it even if you're in the right area. If you have found it, your Pokémon will start with an Exclamation Mark, do a dance swaying from right to left and then have a green musical note over its head. Your character then immediatelly attaches the Shiny Leaf to the Pokémon. As you get more Shiny Leaves, the likelihood of your Pokémon finding one quickly decreases."
Link:
http://www.serebii.net/heartgoldsoulsilver/shinyleaf.shtml
So I guess this is just something else to do once you've done everything in the game, or if you're really bored o_0