set the width to something that is smaller than the viewport (iframe width in this case) and then overwrite it with min-width: 100% to allow for actual width: 100% which iOS Safari by default overwrites.
Responsive iframes in IOS / Iphone
#iframe#ios#iphone#responsive
iframe { width: 1px; min-width: 100%; *width: 100%; // Fix for IE6 }