Acquiring different tree barks in Minecraft

From the minecraft wiki:

"Saplings will only grow into generic trees, never into birch or pine trees, even if the saplings came from the foliage of those trees."

As of version 1.5:

Since Beta 1.5, birch, spruce/pine, and oak trees drop birch, spruce/pine, and oak saplings, respectively, although birch and spruce/pine trees will still drop oak saplings as well. Pre-Beta 1.5, trees only ever dropped oak saplings, regardless of tree type.

Pine and birch saplings have recently been added to the game! You can plant them and they will create a tree of the kind of sapling they are.

According to a tweet by Jens, spruce and birch saplings will soon be added to the game.

Heads-up to texture packers: Slot (15, 3) is now spruce sapling and slot (15, 4) is birch sapling (counting coords from 0)

Presumably, each type of tree will soon drop its own type of sapling, so you'll be able to grow more.