Here's the simple fact: ATI Stream is AMD's GPU-accelerated computing project and for all intents and purposes it is basically AMD-flavoured CUDA. That is, basically, all you need to know. Not, I hasten to add, that this is necessarily a bad thing; I'm a big fan of CUDA and the benefits it brings and seeing AMD GPU-powered system get a similar offering is only a good thing.
Stream should arrive with the Catalyst 8.12 driver package on the 10th of December. Any 4000-series (or higher) graphics chip is compatible, which doesn't give AMD as much backwards-compatibility as nVidia but frankly I doubt that will matter.
To its advantage, AMD has a sizeable selection of developers lined up to adopt Stream come launch - including some pretty big guns.
Adobe is an obvious choice, with Photoshop CS4 and, in fact, the entire CS4 suite, being GPU-accelerated on AMD GPUs, just as with nVidia's.
A less obvious fit is the group of Microsoft applications set to see performance boosts. Vista's built-in Picture Viewer application, PowerPoint (hurray!) and Silverlight are all set to see GPU-accelerated performance increases. The latter being particularly interesting as Adobe's Flash 10, Silverlight's main rival, will see the same benefit.
As an example of the benefits of Stream, AMD is pushing its own AVIVO video encoder. Comparing to Elemental Technologies' Badaboom encoder, AMD claims not only better performance, in terms of both speed and quality, but also a broader range of usage options. And AMD is giving its encoder away for free, too.
Okay, so really I'd prefer to see nVidia and AMD collaborate to create an industry standard for GPU-accelerated computing. After all, both CUDA and Stream are, at base, just compilers optimising multi-threaded programs for the highly parallel nature of the GPU. Maybe Larrabee and its ability to run any x86 language will lead to that once Intel actually launches its GPU.
For now, I'm just glad to see AMD competing on the same turf as nVidia.
For the interested, here are a few more slides.