FYI:
In 1.8 you have to do it like this:
world.setBlockState(BlockPos, IBlockState);
Example:
IBlockState dirtBlock = Blocks.dirt.getDefaultState();
world.setBlockState(new BlockPos(x, y, z), dirtBlock);
or with metadata
IBlockState plankBlock = Blocks.planks.getStateFromMeta(3); //This will be a jungle plank
world.setBlockState(new BlockPos(x, y, z), plankBlock);
Hope this helps you when you decide to update to 1.8 .
EDIT: Read what diesieben07 said.