I've had a few perspectives on passing as my gender identity has evolved. To be fair, passing, though not quite in the sense you mean, started at 5 or 6 years old when I, due to childhood stresses, was forced to conform my behaviors to what my family expected (a boy) and subvert many of my natural feelings. Playing that part as well as I could kept me alive, I do believe, and I can't help but be grateful that I found a part of me that could get through that part of my life. Pushing the rest of me deeper into to the subconcious, however, meant that some internal fracturing had occured, and that the part of me who was present for my childhood grew largely independent of much of the rest of my self.
When I initially began accepting my female self around 19 (after years of severe depression) passing was important to me partly because I wasn't emotionally strong enough to withstand critisism. I needed external validation to help support my internal sense of self, which isn't to say that anyone else could know who I am, but simply that knowing people did percieve me as female helped me ground my identity in the reality of existance as a woman.
As I grew more comfortable with myself as a whole, and got past inital anxieties about my being (as I percieved at the time) a transsexual, I found that in my case absolutely no one had any question about my gender, which I primarily attribute to some particularly fortunate genetics, as well as begining estrogen around 20. By this point, passing became something I put a little effort into. My attention in the realm of appearance shifted to notcing the imperfections in myself and trying to bring my appearance into line with my internal vision fully, much as any person does, trans-* or not.
Along that transition path, I seriously considered SRS most of the time, but I always found some reason not to, usually financial, though I could have managed it if I'd really applied myself to the task. But as I started to finally accept after years of living as my female self, there was simply more to my gender than I was wanting to accept. For years, I had been working on the premise that the female self was the "real me" and the male self was a hollow construct - not "real". About a year ago, I started accepting that my male self might not have been, had my life gone differently, but it had become a very real part of me that I couldn't just discard when I stopped needing it. Following that path for a while has led me to my present sense of being bi-gendered, which is still under very active evolution.
And for Chris, my male self, passing is not easy at all. Those same femine features plus years of hormones have softened my face and given me rather large breasts, which has made it pretty much impossible for him to be seen as male. It's kind of funny, were it not uncomfortable (perhaps annoying is a better word) for him. But along the way, we as a whole have grown stronger, and neither of us feel as sensitive to what others think. Today, we seriously look at finding ways of expression that are comfortable for both of us, which I felt like an awfully uncomfortable idea when transistioning started 6 years ago. Still, I know he needs to find a way of being that people can identify as male, that he be seen for who he is.
As I look at things now, it'll probably be a few more years yet before I find the ways to "pass" as who I am as a whole. Part of that is wanting people to percieve me in a way that's congruent with my internal sense of self, part of it is finding comprimises that satisify my internal sense of self, and part of it is learing to let myselves express themselves fully too, to not feel contrained to one unified way of presenting. It's all another facet of that life long journey of discovering one's self, I suppose. :-)
-Amy (with a few words from Chris)