I feel like there's an expectation for me to dress up more than my husband or any other programmer I know, just to be taken a little more seriously. I don't know that many female programmers that wear tshirts except for promo purposes or at home/super duper casual settings either.
Not to mention most tshirts don't fit female bodies in an ideal way anyway...
I'd say the correlation runs the other way. The best female programmers I've worked with dressed like stereotypical nerds (just like the best male programmers I worked with). The ones who "dressed up" were, by and large, those trying to compensate for substandard programming skills (again, just like the guys who wore suits).
Not to mention most tshirts don't fit female bodies in an ideal way anyway...