It’s available now right here and it’s much more than just a search client — there is now an informational sidebar that sits on alive in Windows. The interesting part is that the informational sidebar does a lot of things that Windows can do for you, but it does it more quickly and in an easier way, begging the question of whether or not Windows itself is really all that important; all of the data for the sidebar is really extracted from the web and your files, not the OS.
I could certainly envision this client running on an old machine that only had Firefox, a free version of Linux, and Open Office installed on it. For that mater, I guess there’s no reason why Google couldn’t just bundle its own OS with those 4 components on it; in fact it would probably run well on older machines.
As with everything Google, there is a Developer SDK available for the sidebar.