As a contrast, consider Apple's adoption of this integrated-OS approach. It can do so because it owns both Mac OS X and iOS. But even Apple is slowly integrating the two OSes, in terms of how it adapts OS and hardware capabilities from Mac OS X to iOS and from iOS to Mac OS X. The fact that Apple is doing it slowly reflects business realities (not undercutting the Mac OS X market and developer ecosystem) and technical realities (not throwing out the software, tools, and services that are widely deployed and depended on today by users, IT, and developers alike).
But as its Mac OS X Lion plans show, it is methodically moving in that direction. Apple and its developers are also greatly helped by the fact that Mac OS X and iOS share a core OS layer, so they're more branches on the same tree than different species, which is the case for WebOS and Windows.
The virtual-machine model
The more likely way to have hybrid WedOS/Windows PCs is by using virtualization. This technology works extremely well, as any Mac user who runs Windows via Parallels Desktop or EMC VMware Fusion can attest. But it's no slam dunk.
Windows is able to run virtualization software to host other OSes as guests. But if WebOS were a guest OS on Windows, there's the issue of how to deal with WebOS's very different UI model and the type of interapplication facilities that WebOS uses, for example, with alerting and communication integration. Certainly any app running in a WebOS VM would have access to those facilities, but Windows apps would not. That means users get inconsistent experiences as they switch apps.
You don't see much of that when running Windows apps on Macs because the underlying OSes have essentially the same facilities. True, Mac OS X offers more touch and gesture facilities than Windows, but the reality is that few Mac apps rely on them. Thus, few users bump into the "why do these gestures that run in Photoshop not work in QuickBooks" kind of issues.
I suppose HP could have WebOS run in its own window, as an isolated virtual machine that interacts with Windows only through shared storage and copy and paste. It would be sort of like using Citrix Receiver for Windows terminal emulation on an iPad (or, as promised, a WebOS tablet when one becomes available) or how desktop virtualization for Windows on a Mac worked in the earliest days. But that's not very compelling -- and simply having your WebOS tablet on your desk next to your Windows PC, with some Wi-Fi sharing would be a more natural, less awkward way for users to run both OSes.