You can’t actually run Photoshop on the iPad or an Android tablet, but you will soon be able to use your tablet to work with the copy of Photoshop running on your desktop.
Adobe recently announced an expansion for Photoshop CS5 that provides a new Touch Software Development Kit (SDK). Developers can use this SDK to create new Android and iPad apps. These new apps will help you bridge the divide between a tablet with the heavy duty work machine, thus enabling you to do more interesting things with Photoshop.
Adobe is hoping that developers will come up with their own apps, but it will also release several apps for the iPad in May.