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.
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. :)