Well there is sort of an app for that.
AirDisplay is a $10 app that turns your iPad into a second monitor. It works with both OS X and Windows. Install the app on your iPad, launch the app and follow the bouncing ball. It will prompt you to download and install the software on you Mac or PC, restart and you'll find an AirDisplay icon in the system tray or on the menu bar. Click that icon and pick your iPad from the list, launch PhotoShop and have fun.
Please follow the directions provided by the developer. I prolly left out some steps.
I tried this with a MacBook running OS 10.5n and a netbook running WinXP. It doesn't work on PPC Macs, and there are some flavors of Windows that it doesn't work with.
I tried the GIMP and InkScape on the netbook and both worked well on the iPad. I don't have PhotoShop on the netbook, but since it's your big iron that's driving the software there's no reason why it wouldn't work. There is a slight lag because of wifi connecting the iPad, but it's not a big deal.
Why would you want to use this? Well, your PC is in the office and your family expects you to join them in the family room occasionally, but your hands are itching to doodle in PS or Illustrator, so now you can do it from the comfort of your lazyboy. Maybe you're a gamer, now you can game on your iPad. Visit those Flash sites. The mind boggles at the possibilities.
This will also work on iPhone and iPod Touch but there is some caution about pixels and retina display not playing nice. The website will tell you all about it.
PS: when your working on the iPad, there is no keyboard available, but both Windows and OS X have a virtual keyboard that works in a pinch.