I've been going through a newly-generated world, traveling across several biomes, and I've seen tall grass everywhere there's regular grass: forests, savannah, plains, etc.
The only biomes in which it doesn't seem to spawn are tundra, deserts, and (of course) the Nether.
Tall grass is incredibly common: If you haven't seen a ton of it already, one thing to keep in mind is that new terrain features generally don't spawn on already-generated chunks, and you'll have to travel to a new, unexplored area to see them.
To confirm this, I loaded an existing world I had which spanned a few different biomes (tundra, taiga, forest, grassland). There was no tall grass in any of the pre-generated chunks. However, I found several instances of tall grass immediately after walking into an area that caused the game to generate a new chunk.
Edit (thanks, Kurley): as of 1.6.6, you can spawn tall grass yourself:
When applied to a grass block, one bone meal is consumed and tall grass, and occasionally flowers, forms around and on the grass block.