I've never really considered the possibility of this in my own life, but I believe that, faced with the possibility, I could definitely see myself dating a FtM transexual. Hell, for that matter, I could see myself dating a MtF transexual as well. And my opinion has nothing to do with my being bisexual, as being attracted to both men and women is not (in my opinion) comparable to being open to dating a FtM or MtF transexual. Just because you like both genders doesn't mean you'd be okay with being in a relationship with someone who has undergone surgery to change their gender.
However, I won't lie, it would feel a little awkward to me if I decided to be intimate with someone who hadn't completely finished their sex change - for instance, if someone who had undergone the beginnings of a FtM sex change operation, but still had lady parts - but I don't think I'd be completely against attempting an intimate relationship with such a person, if I had genuine feelings for them.
Also, as a curiousity question: is it just me, or does it seem like most females are more willing to be, or more open to being with, a transexual (read: FtM) than men are willing to be/more open to being with one (read: MtF)? Just an observation I've made in reading this thread and in hearing discussions about it in the "real world," and wanted to get other opinions if possible.