They're squeeze buttons, used by default to launch Expose. You should be able to change what it does in System Preferences though.
They're squeeze buttons on a mighty mouse but that isn't a mighty mouse. The clear top was what you had on the old optical mouse that was replaced by the mighty mouse. It just had a single button action and the whole top pressed down. Similar to the MM but without the scroll ball, touch sensitive clicking surface and squeeze buttons.
Then why is there an indentation on the side of the mouse? I thought that was similar to the mighty mouse?
anyway, checked wikipedia, and you're right. Just wondering what are those things on the side for.