Part of modesty deals with showing of nakedness and that's defined a couple places in the Bible. I don't see any reason to impose dress standards higher than God's Word describes, except for obvious reasons of weather and practicality.
The other part of modesty deals with that of drawing attention to oneself as someone already quoted above. Keeping that in mind, who would be less modest in America, a normal fashionable dress (on a woman I must add), or the Muslim women that I posted above?