The Null Device
That's all the detail that seems to exist so far. There is a possibility that it's just an elaborate feint; Google could, in theory, have paid McCloud some huge sum to draw a comic to specification, peppered with technical versimilitude, purely in order to send Microsoft/Apple/Yahoo!/whoever's development teams on a wild goose chase. Though I suspect that there is an actual product there. For one, Google are known to use WebKit on Android. More importantly, though, a browser designed as a web application operating system (with the expectations of performance and stability that implies), rather than an information viewer with programmability grafted on as an afterthought (as is the case with current browsers), would line up rather nicely with Google's strategy to make the web into a first-class application platform.
There are no details on what platforms Chrome will run; it is open-source (and other projects, or those willing to fork those, will probably have a field day with this), and the comic does mention Windows in one place, so presumably a Windows version is planned. I'm guessing that Google aren't doing this to help Microsoft sell Windows licences, though, so presumably this is not the only version planned. A Linux desktop version, running on top of X, is probably likely. Another possibility is it running over something lighter than the average Linux desktop, making a robust web-browsing appliance on which the browser meets the conventional definitions of an operating system; either Android or some other lightweight OS.
The other option, of course, is that this is an elaborate hoax, akin to the Photoshopped "spy photos" of new Apple Mac tablets and other fantastic gear that are a regular feature of gadget blogs. The fact that Google's Chrome page doesn't yet exist (at time of writing) does suggest this possibility. Though this would imply that the hoaxers had an enormous amount of time on their hands, excellent comic drawing skills and an uncanny mastery of the drawing style of Scott McCloud.
Update: Google have confirmed Chrome. It's initially a Windows product (presumably to win market share before IE8 comes along and shuts off Google's oxygen with its advertising cookie blocker), though Mac and Linux versions are in the works. The Windows version will apparently be out tomorrow.