A symbol of a lock inside of a circle with an arrow has suddenly appeared to the left of the battery percentage indicator on the top right of my iPad. Does anyone know what it is?
Are you using the iOS 4 beta? This function does not exist on the 3GS using iOS 3.1.x as far as I know.
If you are using the iOS 4 beta, pressing the home button twice should bring up the task switcher. Then scrolling either left or right should allow you to see the toggle button to lock the screen orientation.