While the web weenie in me is extremely excited about the news of a Google web-centric OS for mobile devices, I think that some of the tech press is off in their assertion that Chrome OS in some way will threaten Apple. I don't think Chrome OS will impact Apple in the notebook space anymore than Android has threatened their smartphone dominance. Neither product is targeted at Apple's consumers.
Apple sells a polished digital media experience and targets consumers who want to pay a little more for the quality, simplicity and polish. Google's Chrome OS won't target that same consumer. Google's offerings are all fantastic for what they are - free and good-enough to get the job done. They aren't in any way a threat to Apple who's customers feel that the premium price they pay is an assurance of quality and value. Besides, Google Docs are great for quick and dirty documents but they'll never provide the impact of a Keynote presentation or the pro feel of a Pages production. The web simply doesn't offer that level of software robustness today and won't anytime soon. Apple's customers care more about multimedia than they do productivity tools and Google's offerings simply can't offer the same level of streamlined sophistication or integration that the iLife suite does for managing and creating digital media.
On top of that, Apple has the advantage of deep hardware integration that augments their software in ways that won't be possible with a distributed, open-source OS. Web service integration at the OS level is relatively easy to achieve but it's not the whole story. A truly immaculate computing experience, which is what Apple customers demand, can only be achieved with hardware and software integration.
It's Microsoft that should be most concerned about this development simply because they sell software as their primary source of revenue. Google will give this OS away to anyone that wants to put it on cheap netbook hardware. These budget-conscious "laptop hunter" customers that Microsoft counts on would probably opt for the Chrome netbooks. I don't see Microsoft being able to counter this without completely altering their business model.
What about Ubuntu? While Ubuntu is a perfectly suitable OS for netbooks, it's not the most usable and won't offer the same level of service integration that Chrome OS will. Simply put, it doesn't feel like a product and I don't think it stands a chance against an actual brand.
Ultimately, both Microsoft and Google should both be concerned about Apple's missing netbook. I think we've seen what they can do when they take their time and truly let design happen. They could very well be about to deliver a product that redefines mobile computing in ways that will take the entire computing industry down an entirely different path.