Grass

Tall Grass is an upcoming feature in 1.6. It will spawn on Grass blocks in certain biomes. Seeds will only spawn from harvesting these blocks. Little else is known about it however. Based on the picture provided, tall grass will not be a solid block, but more like an aesthetic weed that would get in the way but would not block movements, like Sugar Cane and Wheat.

The color of tall grass will be biome dependant, and the same color as grass.