A pretty garden
Currently As for Non-Shrooms The decoration blocks exist purely as embellishments, and currently serve no function. They may only survive on a dirt block and, much like grass, will disappear without a source of light. You are unable to stand on a decoration block, but you may walk through it freely with no hindrance to movement. Their only purpose currently is decoration.
Currently As for Shrooms: These blocks only survive in the shade, on stone, cobblestone, and gravel. Sunlight or being on a different block type will kill them, and these are used in survival mode to affect health.
Currently as for Both: They can also be used to block water and lava. Water will not pass through them while the player still can.
Without sponge blocks, the decoration blocks can be used to fill the space the player is standing in when filling a flooded area in a confined space, after the rest of the water blocks have been filled in.
The tree sapling decoration will grow into a full tree in Survival and Indev modes, and more can be harvested by chopping up leaves from mature trees.
Also painting blocks have been added to indev since 2/23/10.They have no purpose and are merely decorational.