It will only allow you to set one of the built-in searches as default, and even if you change the default Speed Dial will continue to show Google. You can hide the speed dial search though. Go to Settings, click outside the search box, and on the keyboard press up, up, down, down, left, right, left, right, b, a. Near the bottom of Settings you will now find an option to hide the search, as well as several other formerly hidden options like setting an exact size for speed dial thumbnails.