That really depends on the dress. Dresses can range anywhere from the peak of formality (equivalent to white tie attire for men) to somewhere in the neighborhood to nearly the level of informality as athletic or sleepwear, and anywhere in between.
Eh. "Accepted" is probably a relative term here. As in, she was probably not "accepted" the way a man would be in casual attire. She just had slightly fewer hurdles to jump.
In any case, the point here is that men's reactions to women in dresses often cause distress and make women feel uncomfortable in technical settings. That's frankly ridiculous, since (as far as I understand) many women like to wear dresses and don't feel that they're formal. Maybe it's due to a misunderstanding? Men labor under the misapprehension that dresses are formal attire when they're not, and that causes the disconnect, etc. etc. Doesn't mean we should just accept it -- let's correct the misunderstanding. :)
FWIW (and I know I'm beating a dead horse), I did an informal survey on Facebook about this. The question was, simply, "Are dresses formal attire?" The universal consensus is "no." Dresses can be easier than pants, comfortable like pajamas, etc. So I think there's a good chance that many men simply have the wrong idea about dresses and they're the equivalent of jeans & hoodies.
Anyway. Maybe it's worth asking the women you know.