The shoe that Google’s been itching to drop – the one everybody knew was dangling – has finally dropped.

With Pearl Harbor-like timing of a declaration of war, Google VP of product management Sundar Pichai and engineering director Linus Upson said in a midnight blog post Wednesday morning that Google’s going to develop a fast, lightweight, open source operating system based on its nine-month-old Chrome browser to compete against Microsoft.

The so-called Chrome OS, described as Chrome running in a new windowing system on top of a Linux kernel, won’t be ready for prime time until the second half of 2010, a lifetime in Internet minutes.

The schedule gives Microsoft, which is busily detaching its browser from its operating system in Europe, plenty of time to figure out how to retort if it doesn’t already have a plan other than Windows 7.

Google’s aiming to put the thing on ARM- and x86-based netbooks first and sometime after that on full-sized desktops, both places where Microsoft overwhelmingly dominates. Google says it’s working with “multiple OEMs to bring a number of netbooks to market next year.”

It later identified them as Acer, Adobe, ASUS, Freescale, Hewlett-Packard, Lenovo, Qualcomm, Texas Instruments and Toshiba.

As open source Google says Chrome OS will be free, a clear attempt to drain Microsoft’s treasury.

Chrome OS is not Android, the Linux-based operating system Google is pushing onto phones, although the two admittedly overlap since Android is also supposed to fit into netbooks.

Instead Chrome OS is described as a new Linux project that will be open sourced later this year so the community can contribute.

Google says existing operating systems, meaning of course Windows without saying so, were designed before the web was invented. The web and applications running in the browser – not the operating system – are central in Google’s thinking. Chrome OS is supposed to get the user onto the web in seconds via a minimal interface designed to stay out of the way.

The company says it will “completely” redesign the underlying security architecture to avoid viruses, malware and security updates. (Cookie-free might be more useful at this point but unlikely coming from Google.)

Google promises that all web-based applications will automatically work and any new applications written for the thing will also run on any standards-based browser on Windows, Mac and other Linux operating systems “to give developers the largest user base of any platform.”

Google uses Linux internally so it can presumably make an operating system robust enough to hold up. But netbooks are not the stuff of mission-critical work, Linux has so far failed to loosen Microsoft’s grip on PCs (not even on netbooks), and the Chrome browser has yet to make much headway against Microsoft or even the Google-subsidized Firefox.

By Google’s count it’s only used regularly by 30 million people, a relative rounding error.

Despite its talk of community development, which is probably more viral marketing than anything else, an operating system is also going to cost Google a pretty penny or two to extend, support and maintain. (Ask IBM. Its futile OS2 efforts cost it close to a billion dollars a year back in the day.) And Google might actually have to talk to people to support the thing, something it has no cultural skill at.

Heck, it might even have to learn how to write a press release – or talk to the press. A blog posting is not marketing.

The object of Chrome OS, aside from harrying Microsoft, which is trying to harry Google back with its new Bing search engine, is of course to sell more ads by driving traffic to Google Search and other Google “services.” It is not about getting work done.

The Guardian reckons Chrome OS will “strip whatever hardware it runs on of most of its usefulness, without actually reducing the price by very much.”

Tim Negris, the former IBM and Oracle VP who invented the expression “thin client” for Larry Ellison when Oracle and its pal Sun Microsystems – Google CEO Eric Schmidt’s alma mater – were pushing the initial concept of a cheap network computer, takes the Chrome OS for a poor play on that last century idea.

He gives Google “all props” for its success so far but calls the Chrome browser “a fat client in a girdle, sneakily sucking cycles off the plates of other (non-Google) apps” and says that “even if that weren’t the case, it will still probably make a crappy app platform. A largely stateless, page-oriented interface is not the best way to interact with most computer applications, other than stateless page-oriented ones like web sites. Windows, Linux and Mac OS (Mach Unix) are far from perfect and I am a big believer in the concept and manifest destiny of the thin client, but, for Google’s particular concept to work, the Internet itself must become an operating system, and not simply be a means of linking one operating system instance to another. Google is taking the web as we know it, including 2.0, as far as it can, but it’s not in a position to drive the changes needed to make the true always-on thin client a reality. That task is in the hands of Cisco, Akami and others who can make the connective infrastructure much smarter than it is today, and that is what it will take to make the endpoints as dumb and simple as Google would like them to be.

“The Plan 9 and Inferno distributed operating systems work that was done at Bell Labs some years back was a lot closer to the right approach than the dog’s breakfast of network computing being served up nowadays by Google, Microsoft, Apple, the Linuxes, Oracle and other ‘end point’ vendors. The Bell stuff was not perfect or complete by any means, and probably flawed in places, but that’s not why it didn’t go anywhere. Plan 9 was smothered in its crib and Inferno sent to a foster home by their impatient, unloving parents on the business side of AT&T. Google would do well to pocket its pride and reach out to people like Rob Pike, Ken Thompson, Dennis Ritchie, Brian Kernighan, Bjarne Stroustrup and others who know history well enough to avoid repeating it.”

Google is ratcheting up its challenge to Microsoft elsewhere too. Hours before its Chrome OS disclosure, it finally ripped those perpetual beta labels off Gmail, Google Docs, Google Calendar and Google Talk to appeal to a wider swath of the business set.