Cathar has said he will be developing a GPU block of the Cascade design. But it will be some time before it's out.
He's swamped right now with getting the Cascade CPU orders shipped to those on his waiting list (Me to!).
Think about the number of really high performing GPU water blocks there are. Next to nothing compared to CPU block development. When the GPU Cascade comes out it is going to sell like crazy, nothing else will come anywhere near it.
Present GPU's don't develop the amount of heat a CPU does, even if overclocked hard. But they are getting hotter fast with each new card series. The NV 40 or Ati 420 will most likly need a block like the Cascade to overclock well.
AND WE'LL HAVE THEM JUST IN TIME ! !