This is a weird one and I'm not sure why it happens, but it's happened to me twice when messing around in the menus and going back to read. Basically, the top bar disappears! Wha? I fix it by hitting "Back" and then going back into the book again, but look - I actually gain an extra line of text!
Does this mean Amazon is planning to let us hide any bars in the future? Seems rather odd...
PS Ignore the terrible picture colors. It's my cell phone that blows.