On the Desktop I see the keyboard icon in the tray, but in the Start screen there is no such icon. How do I bring up the on-screen keyboard to search using touch in the Start screen?
When you swipe from the right (to bring up the Charms bar) and select the Search charm, you get a text box. Touching it should show you the OSK.
Source: Microsoft's site about the Surface. Can't test this myself since I don't own a Surface, but maybe it is of help.