"A woman must not put on a man's apparel (beged ish), nor shall a man wear woman's clothing; for whoever does these things is abhorrent to the Lord, your God." (Deuteronomy 22:5)
What does it mean that the Torah contains what's generally considered to be an injunction against cross-dressing? Certainly, it can be and has been read as an attempt to create and enforce gender roles (and, possibly, the power structures accompanying them). But as with everything in Torah, its true meaning is more complex.
In attempting to understand this biblical injunction, we may need to begin by ascertaining what exactly is "men's clothing," or "women's clothing." One-hundred and fifty years ago, a woman in pants was downright shocking. These days, even women in boxers don't seem to be defying our Western concept of gender. The famous 11th century commentator Rashi pushes the question further (in his commentary to a talmudic passage, Nazir 59a), arguing that the passage really intends to safeguard against adultery — that is, to prevent a woman from passing as a man to facilitate immoral behavior. While it's nice that he differentiates between cross-dressing and immoral behavior, one wishes he could see the way he foisted women into that same old role as temptress.
All this said, the beged ish (man's clothing) question becomes more complicated as we begin to consider the Pandora's box-o'gender. While the issues raised by the modern transgender movement have different implications for the lives of many of our fellow human beings, the questions such as What is a man? What is a woman? and How do we know? are ancient. The rabbis of the Talmud recognized as many as seven genders and debated the status, responsibilities and roles of each one. But what the rabbis of the Talmud did not anticipate is the phenomenon of more and more people who argue that gender can be chosen. This can mean anything from "What we've been taught about 'girls' and 'boys' is a crock of hooey" to "I use words that resonate with my internal knowledge of myself," or, yes, "I need a change on the biological level." But not even those of us who identify either as male or female — and are comfortable and happy with that identification — embody "perfect manhood" or "perfect femalehood."
Our conceptions of our own gender shifts and changes over time. Even for folks who don't identify as cross-dressers or some shade of transgendered, the clothes we choose to wear — say, nail polish? combat boots? both? — can be potent tools for creating and reflecting our ever-evolving and nuanced understandings of our own gender.
All of which makes the Torah's prohibition against cross-dressing either more or less thorny, depending on how you look at it. Some could say that since we now understand how complicated gender is, the black-and-white model used in Deuteronomy 22:5 no longer applies. But perhaps there's more to it. Maybe what the Torah suggests is that we should all be true to ourselves and represent on the outside, with our clothing, what's happening within. That is, however we define our genders — whatever they may be — we should wear 'em, and wear 'em loud. If pretending to be something you're not isn't "abhorrent," I'm not sure what is.