Just to let every one know that you should check up Installer.app and check this new app out. This app is its initial stage now, but still you can draw and erase. Erasing is done by shaking the iphone, completely erasing everything. It's icon is an etchasketch, so I can understand the shake/delete thing.
This is an alpha release for this apps. When you run this app for the first tme then it might slow your iphone for a bit and if you want to avoid any freezes/hangs ups, you must restart your iphone after installing the Sketch program.
You can go here if you dont have the installer :
http://sketches.latenitesoft.com/Home.html