I just found something by accident. Open a page in Safari and follow some links so that you have some history for a tab. Now, place your finger on the edge of the iPad but off-screen. Slide your finger across the face of the device (from off-screen to on-screen) and watch what happens in the tab. This only works when a tab has some history, e.g. you've followed some links in the tab. I had no idea you could go back and forward in tab history without using the arrow keys at the top left of Safari. Nice feature, that.