I set the max width for my video to be 100%. On phones the video automatically fits the width of the screen. Since the embedded video is only 560px wide, I just added a 10% left-margin to the iframe, and put a "0" back in for the margin for the mobile CSS (to allow the full width view). I did't want to bother putting a div around every video...
Desktop CSS:
iframe {_x000D_
margin-left: 10%;_x000D_
}
_x000D_
Mobile CSS:
iframe {_x000D_
margin-left: 0;_x000D_
}
_x000D_
Worked perfect for my blog (Botanical Amy).