Since the introduction of the Mega Taiga biome in 1.7, there has been a dirt block with the data value of 3 which does grow grass. This block was for some reason removed in the latest snapshot and it causes all pre-existing Mega-Taiga biomes to display the missing block texture in absence of the dirt.

Mega-Taiga biomes created in the snapshot substitute the standard dirt block instead to avoid this problem, but I'm unclear as to why the data value was removed in the first place.