UPD: Actually, the blocks was replaced, but the client didn't received that information.

Don't know how to describe it only by words.
I wanted to make a snow only on top of the mountains.
Here's a part of my config:

SurfaceBlock:78
ReplacedBlocks:78=2(64-100),78=0(101-140)

Now see the screenshot:
http://pikucha.ru/i9uul

What TC/Bukkit versions you use ?

TerrainControl v2.2.4 and Craftbukkit 1.3.2-R3.0.
Maybe i should try to rollback to CB 1.3.1-R1.0... But everything else works fine.

I believe Minecraft finally fixed this: chunks are not sent until the terrain is fully populated. ReplacedBlocks doesn't have this problem anymore.