Rumors have been circulating the Interwebs for weeks now about when Chrome OS will arrive. In fact, a beta version of the operating system was expected to appear as early as this week. That may still be a possibility, but given the wording of the Google invitation, I wouldn't get your hopes up.
Google is billing Thursday's event as "an update on our progress with Google Chrome OS." The event will include a demonstration of what Chrome OS is and how it works. Google also said it will detail its "launch plans for next year."
That last quote makes it pretty clear that the operating system won't be ready until some point in 2010. Hopefully it will be earlier in the year rather than later in the year.
Sundar Pichai, vice president of product management and Matthew Papakipos, engineering director for Google Chrome OS, will be leading the event.