There's been a way to do this on the Mac for a while, using a $10 app called Air Display. Not being a Mac guy, I can admit to having been somewhat jealous of this capability. But now there's MaxiVista, a (similarly priced) program that does the same thing for Windows user.
For $10, you get an app to install on your iPad. Also install the free companion program on your Windows PC, and make sure that the computer and iPad are connected to the same network. The Windows app should automatically find your iPad, and you're in business.
It works pretty effortlessly - the iPad automatically appears as a second monitor when you run both the PC and iPad apps, and you can seamlessly drag stuff between the displays. When the desktop app is running, you'll lose Glass (in Windows Vista and Windows 7, obviously). And the display is understandably a bit sluggish - I wouldn't try watching Hulu on it, but otherwise it's perfectly adequate for Web browsing and other mundane tasks, as you can see in this video:
One feature request for these guys: Find a way to let us use the iPad as a touch display, so you can pick it up and manipulate windows and links the way the iPad begs to be used. Give us that, and it'll truly be worthy of its $10 price tag.