During the unveiling event, Google promised that Chrome OS would be ready to by the end of this year -- before the holiday season. It looks like they'll be able to keep that promise, as bug comments on their Google Code site for the project indicate that the OS has already hit "RC" status -- the most recent build would seem to be 0.9.78.1 (or RC 78.1). When that number evens out to "1", we can probably expect Chrome OS to be ready to go. Further, the most recent updates in this thread from three days ago twice have a Google employee referencing a Nov'11 date.
"We'll push this after Nov"11," the employee writes in response to a question about a specific feature."
Here's Google's statement: "We're very happy with the progress of Google Chrome OS and expect devices will be available later this year. We'll have more details to share at launch."