A second monitor for your Mac, when you're on your Grownup Computer: That's a perfect use for the 1024x768 iPad, and exactly what iDisplay lets you do. But it needs some work.
The concept and execution are equally simple: Your iPad serves as a secondary display for your Mac (and only your Mac), by leveraging a transparent VNC server and an artificial second monitor. Run the server app on your computer, run the client app on your iPad, arrange the displays in your OS X settings, and you've got an instant second monitor. Sort of. The catch(es):
• If you don't set it up right, it doesn't work. Or worse, it works, but it warps your Mac display resolution to the iPad's, which is dumb. So: Make sure you turn off mirrored displays in your OS X Display settings first.
• The server autodetection is shaky, so you might need to enter your computer's IP address and access port manually. Just hover over the iDisplay Menubar icon in OS X to reveal both.
• It's slow. Sometimes it's bearably slow; with a solid connection, it's choppy but mouseable. Other times - and on our test network, most of the time - it's VERY slow, which relegates the second screen to passive tasks, like displaying Twitter feeds or IM windows or something along those lines. We'll have to see how user reviews pan out on this one, since different networks will give different results, but the app was slower than full-on VNC servers.
It's buggy to be sure, but the promise here is massive. And the app is developed by the same folks who make IM+, so we can realistically expect for most of these bugs to be fixed. [Gizmodo's iPad App Review Marathon; Gizmodo's Essential iPad Apps]