"Google Chrome OS is an open source, lightweight operating system that will initially be targeted at netbooks."
There's more from John O'Conner's Blog:
"Google Chrome OS will be a slightly more beefy Android OS ..."
And from the FAQ session of the main site, there's a screenshot of this OS (this image does not belong to me):
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Personally I think it's still too early to judge the impact of it towards Windows OS on desktop.