Let me challenge everything Choster wrote above. Mobile sites are still necessary even if you use a responsive design. For the reasons mentioned above by Marshall, you have to limit the number of elements downloaded by mobile users even if you offer them a responsive design. Responsive designs are all the rage now, but they are not a perfect solution. You can through a responsive design limit some elements, but chances are that you'll still download huge js files and the likes to make your responsive site work.
On many projects I've worked on, we kept the client's mobile site because it was just that optimized and still a better experience for users.
There are other problems with responsive sites, even if they currently are the darlings of the Web development community. I like to be contrarian but test it out for yourself. Resonsive is not a one size fit all.