You know, OS X is just an Apple-branded FreeBSD distro.
The question is whether Google redesigns the graphical interface (replacing X Window with their own system) like Apple did, or if they simply use X Window to display the Chrome browser. The latter solution would be easier in terms of 3rd party support for audio/video, since companies can reuse their linux code. Of course such a solution could be upgraded to a full Linux desktop with little extra work.
But as I understand Google's claim, they want to produce something more radical than that. If they build a new architecture on top of the linux kernel, a lot of web-based software needs to be written to rewritten before the Chrome-OS becomes usable. This might turn into the classical chicken-and-egg problem that companies don't want to develop for a non-existing market because the users don't find the apps they need. We will have to see how it turns out.
Google has been dabbling behind the scenes in this for a couple of years, and if the clues other companies have been leaving are any indication, this could be a radical OS.
First, there are companies playing around with "instant on" OS, by including Linux (or at least a portion of it) in the BIOS. You power on and in a couple of seconds, you're ready to go online. A remarkable screenshot is here:
http://news.cnet.com/8301-17938_105-10110716-1.html?tag=mncol;title
Then, there's Google Gears, where you can have Google apps cached locally when online access is not available. And Google Gadgets for Linux for snippets from the web (gOS also features this):
http://www.thinkgos.com/gos/index.html
With everything done from the web, you can go with a smaller hard drive (or no hard drive at all). And, if Google makes money on search and advertising, they could give away the OS if they want.
Suddenly, the margins-strapped netbook market looks a lot more profitable for HP, Dell, etc.
The big question is whether consumers would go for it, but if you have a decent, very portable machine for $300-$400 with Google's name on it, designed primarily for web access, I think people will give it at least a look and take it seriously.
Also consider that Google already has their Chrome browser available for Windows, and is working on a browser for the Mac and Linux, they may be able to sneak some of the features in the upcoming Chrome OS to the machine you already use.