@Draug, sure behaviour has some environmental impact aswell, but 4-6 years olds expressing their gender where it is different from that which was assigned at birth is not something they or their parents just choose.
Gender identity is hard-coded at an early stage of development, just as sex organs are hard-coded at one point, and sexuality (though it does not usually express itself until puberty) how we express these three things varies with culture and upbringing - how a penis actually looks like will depend on things like circumcision, but that is just a superficial difference, like the influence society has - society may define gender norms, but the brain is hard coded to identify with one or other (or none or both) of those genders.
And no Draug, most trans people i know WANT the law to recognise their trans gendered identity, it takes a lot of trouble (in Ireland) to get your passport changed from your assigned at birth gender (that is your sex) to your identified gender. Some countries (and states) don't allow this at all.
It is pretty important what legal recognition they can get, for things like marriage (though that will become somewhat less important if gay marriage laws are passed)
And as Tergem said, it is 18 months to 2 years of living as your identified gender on Hormone replacements before a doctor will consider bottom surgery (srs).
I don't have sources, i'm basing this on personal conversations with individuals who have been dealing with these issues in Ireland. But aside from Hormone replacement (which is not as simple as it sounds, can have nasty side-effects, apart from the time it takes to kick in and the learning required to deal with your new emotional responces to things) there is also laser surgery to remove facial hair, facial reconstruction to change the shape of the face, and vocal cord modifications (whose names escape me, but to help alter the sound of the voice) all add up to a pretty expensive bill.
And along with that, female-to-male bottom surgery (srs) can build something like a penis, but there will be very little sensitivity in it, and the resulting appendage isn't the most useful for things like sex... (and with strapons available it is more than likely that expensive and potentially dangerous surgery will be avoided by these men who can get as much sexual pleasure with partners without the surgery...)
So Draug, assuming a trans-woman is living as a woman in her day-to-day life, has not been outed in her community, has breast implants, and passes effectively as a female but has not turned her penis inside out (outside in?) you think she should be forced to use a mens room because the cubicles in the womens room might offend someone?
As for asking me to back up my claims, I don't have any links to evidence. I haven't done the research myself, nor read it. But as has been pointed out, penis weilding bad guys can enter women's restrooms REGARDLESS of the law. This does nothing to protect people it is intending to protect and it harms those trans people who should be innocent until proven guilty, right?
Also Krellin, Draug, please explain to me, what is with your massive obsession with penises in this whole conversation, why does the whole world revolve around them? The identity of people, their place in society? It is all about the penis, the only qualifying factor, the only thing to fear (when younger kids will pretty much accept what they see if it is explained to them, and not be freaked out in the slightest) There are many dangerous people in this world who i would not want near any of my kids, but what is between their legs has very little to do with it, what is between their ears is the worrying thing.
(this whole thing reminds me of this: http://www.ted.com/talks/alice_dreger_is_anatomy_destiny.html but i'm not sure why)