I believe Svein is thinking of render-to-texture which indeed could be used to emulate Stream-Out functionality (but with reduced performance of course). This is also hardware specific and was still not supported on the first GPU's.

From: Sebastian Sylvan [mailto:sebastian.sylvan@gmail.com]
Sent: den 4 november 2008 20:06
To: sveina@gmail.com
Cc: Tobias Bexelius; haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Memory efficiency questions for real-time graphics

On Mon, Nov 3, 2008 at 3:45 PM, Svein Ove Aas <svein.ove@aas.no> wrote:

On Mon, Nov 3, 2008 at 11:31 AM, Tobias Bexelius
<tobias.bexelius@avalanchestudios.se> wrote:
> Before Direct3D 10, its too costly to read back the updated vertex data
> in every frame, which force you to make this kind of operations on the
> CPU.
> With D3D 10 however, you should use the new Stream-Output stage which is
> used to return updated vertex data directly to a vertex buffer on the
> GPU. So if you can afford a new graphics card and likes Vista, that's
> the way to go :)
>
Or you could use OpenGL, which has supported that since the first GPUs
that did were released.