Because of the prevalence of the problem, the Aruch ha-Shulchan(7) ruled that in a locale where the majority of married women do not cover their hair, we can no longer consider hair an ervah. In his opinion, only in a locale where most women keep their hair covered can uncovered hair be considered an ervah. This controversial ruling was accepted by some poskim(8) and strongly rejected by others(9). Harav M. Feinstein(10) ruled that one can rely on this leniency only under extenuating circumstances.
8 Ben Ish Chai, Parashas Bo 12; Seridei Eish 2:14; Yabia Omer 6:13.
9 Mishnah Berurah 75:10; Chazon Ish O.C. 16:8 and most other poskim.
10 Igros Moshe O.C. 1:39,42,43; O.C. 3:23,24; E.H. 1:114.
A very logical extension of this idea would be the following from Judge Richard Posner in
Nudity as titillation or outrage is relative rather than absolute. In a society in which women customarily go about in public bare-breasted, there is no shock value in a bare breast, while in Victorian England, where decent women were expected to wear dresses that reached from the top of the neck to the floor -- where even the legs of furniture were sometimes clad for the sake of decency -- a bare ankle was a sensation. Since then female dress has become progressively less modest, and today many decent women appear in public in states of undress (mini-skirts, hot pants, slit skirts, body stockings, see-through blouses, decolletage becoming outright topless evening wear) that would have been considered nakedness, or the garb of prostitutes, thirty years ago. A striptease that ended in a degree of nudity no longer suggestive of preparations for sex -- a striptease that left the stripper garbed as she might be for an expedition to the supermarket -- might lack erotic punch today.
Obviously the Poskim above would not hold this way. (If I am not mistaken, they distinguish between hair which is only a sort of Ervah and actual flesh which is intrinsic ervah.)
However, it does raise some interesting questions concerning the outer boundaries of the halachos of Tznius, how much of the halachos of Tznius were written by Poskim based on the societal mores of their times (eg the Rambam insisting that woman should wear veils (Tzeif) perhaps using Rivkah as his source but almost definitely influenced by the Muslim culture that he was surrounded with), and how should Halacha change based on the societal mores of our own time?
An interesting bit of backlash against Jewish fashion law almost lead to the severance of the great community of Altona Hamburg Wandsbeck. A Jew by the name of Getting passionately declared his unwillingness to give up on his wig  in the following letter/blog predecessor:
I cannot deny, that as a true Jew, the behavior of the
community, which can hardly be defended in the face of the
considered judgement of our nation, makes my heart sick.
Therefore, I wish to have no further formal religious
association with them until it pleases them to alter the
intention [of the law in question regarding the hair bag]. I
know that I have acted virtuously and cannot prevent such
entirely unexpected consequences. As long as there exists no
formal association between us, it is natural that by no means
will I touch the [financial] burdens of the Community with
even the tip of my finger. When I want to be charitable
towards my brethren, it should happen according to my
conscience and not according to the regulations of a contract
between us, which has been annulled. Therefore, I formally
protest, until such time as I wish to reunite [with the
Community], against all matters and undertakings, fines and
the like, which the Community is authorized to inflict for the
sole reason that, and only for as long as someone is
recognized as, a member of their Community.
That is because it was in my civil life that I wore a hair bag
Furthermore, I want whoever was responsible for the errors in
this matter to admit it and show remorse. This I ask in virtue
of the high esteem and affection with which I am
Moses Joseph Getting
29 April, 1767
Note especially the resentment against the Rabbis for involving themselves in his "civil life" - a very modern sentiment.
The entire rather strange episode is documented in Horowitz, D.. Fractures and fissures in Jewish communal autonomy in Hamburg, 1710--1782
 Apparently this - Getting had been fined for wearing a then highly fashionable silk wig adornment, known as a Haarbeutel or "hair bag." The Brockenhaus Lexikon describes it as "usually a little black silk or taffeta ribbon which sits flat on the upper back, containing the locks of hair down the neck, bound with decorative silk cords."312 The Haarbeutel came into fashion, from France, in the early eighteenth century, and by the time Getting wore his it had "gradually won favor in the most elegant salons"313