Hey, you with the PC! Instead of sitting there sulking because iPad-as-second-monitor app Air Display is Mac-only, you might want to give MaxiVista a shot. It's the PC equivalent, it works wirelessly, and for now it's your best/only option.
MaxiVista's only been out for about a week now, and at $10 costs the same as Air Display. You can orient it to the right or left of your PC (or above/below, if you're feeling frisky), and it's conveniently compatible with both 32 and 64 bit versions of Windows XP, Vista, and Windows 7. The video drivers are compatible with the WDDM graphics card standard, and the display frame rate is said to be very close to that of a conventional monitor.
While it looks great in the promo video above, some caveats remain: early App Store reviews indicate that Aero doesn't work with MaxiVista, and video compression quality is a bit of a question mark. But if you own an iPad and are looking for an extra wireless monitor for your PC, you can't find a better option right now. Or, you know, any other option at all. [MaxiVista]