What about a netbook running an Atom processor and ChromeOS? Starting to get shaky a little, but sure, I bet the market will count these.
No, and it's shaky at all. ChromeOS is not an operating system in the traditional sense that lets users install any application they want, play any media they want, get direct access to the hardware, etc. It's a stripped-down OS; a thin internet client that will run some apps but is not a general-purpose OS.
Can I build and install any application I want to on this generic OS, getting direct hardware access, having rights to play whatever media I want on it? If so, I think it's a PC, regardless of what processor it has, whether it has a touchscreen, etc.
ChromeOS would satisfy your build and install application piece, as you can build an HTML 5 based application and install it. Can it run any application? No, but neither can any other OS, they run what the OS can support. The other question here is how you define an application. Does a pile of HTML and Javascript code count? Some say it does, others do not. I'm not sure about direct hardware access, but here it also depends on your definition. ChromeOS will support WebGL, so the HTML5 apps will have GPU access, similar to Direct3D apps on Windows. Certain desktop OSes counted in the PC market also deny direct access. Media wise, ChromeOS will also allow local or remote playback of your content, or others. Thats why I said it's shaky, as even the definition on the desktop/laptop side is shifting a bit.
Bruno, I brought ChromeOS up, as it's potentially the one that will blur the PC/not a PC argument even more, since it can be on an x86 system/netbook, an ARM netbook, or even the iPad and other tablet form factor devices. I'm sure in any case, the analysts will be squabbling about this for a while, either when it involves the iPad running iOS, or an ARM netbook offering either Android or ChromeOS.