Vegas has never supported GPU-enhanced rendering or preview.
I suspect it would take a major redesign of the core architecture to make this work.
I have not heard of any development plans or even any rumors that this is being considered.
I'm using an nVidia 9600GT and the $30 Badaboom h.264 encoder which uses the GPU of the video card. It absolutely SMOKES any other piece of h.264 encoding software I have on my Quad-core. Its roughly 7 -10 times faster than encoders without the GPU acceleration. Badaboom is limited in the number of video formats which it will accept*, but for those that it does, it screams.
Of course, there is absolutely no benefit within Vegas. It really would be a GIANT leap forward if Vegas made use of this GPU horsepower. It's at the very top of my Vegas wish list.
nVidia has more companies using CUDA for film/video applications than ATI does for its stream technology I believe. Or rather, I don't know of any companies using ATI's solution.
One could also program video shaders... this will work on both vendor's cards, but it is not as fast I believe.
2- A lot of high-end systems like Flame, Da Vinci Resolve, etc. are using GPU acceleration. Before that, Flame ran on expensive SGI systems and Da Vinci on specialized hardware.
That (an encoder or codec) seems most likely the first place it'd appear in Vegas. You'd render to the Cuda or Stream enabled encoder and then that uses the GPU.
Nobody knows what SCS is going to do. There's a need to move away from VFW and perhaps when they do it something like this could be enabled for Vegas' preview window if they're really ambitious (which they usually aren't).
The other possibility is for GPU processing to get built into Windows in a standardized way, which would make it much more likely for Vegas to support it in its next ten year plan.
I was looking at the Resolume website last night and they had a bit of verbiage about why they completely rewrote the application because DirectX wasn't cutting it for them. Neat VJ toy for those of you interested (not you, Cliff. Different kind of VJ.) It made me think of Vegas' problems with VFW.