I'm sure I just haven't looked hard enough, but I'm surprised nobody's put a few of those level shifters in an arduino shield yet, to convert the whole board to arbitrary voltages. The analog pins might be a little harder.
Level converting is surprisingly difficult when you have to deal with both push/pull outputs, digital&analog inputs, and pullups for open-drain buses. In fact, I am not aware of any level converters which can deal with all of that.
