Now that I am going out more often with gender-correct presentation (leaving the man clothes behind) and getting better with makeup, I seem to be passing more often, and I can definitely empathize more with the original post. Before, I really couldn't pass at all, now I get barely a second glance as I walk into the lady's room (and my wonderful girlfriend reminds me that a lot of those second glances are because I am beautiful and tall

I love her so!).
But last night, shopping for bras, a lady from the women's department walked up to me to tell me I had a sticker on my sweater. I thanked her (my voice is still not passable without tons of effort, and I was tired). She then asked me if I ever went to <insert gay bar here>. I told her "occasionally" and she got an "I knew it!" look on her face and walked off.
I could have been devastated, but then I stood back from myself and thought, "OK, so here this woman has had at least sixty years of being female, and *she* wasn't sure about a 6'4" woman with an adam's apple the size of a walnut. I must be doing *something* right!"
Still a bit shaken, but I was able to take some hope from the experience...
And now I, too, should probably start working on prepared answers...

Thank you so much for this thread, Suzi... so much great discussion has come from it!