Look, I believe that there's a scientifically knowable basis. I just have some problems with the implications that some people draw from that.
First, back to the original topic of this thread: having a scientifc explanation doen't make it a mental illness any more than a scientific explanation for homosexuality makes that a mental illness.
Second, I don't think we have a very good explanation yet.
Third, I hate appealing to science to justify my own existence, basically because I don't buy the resoning myself. being transsexual is okay because it's okay, end of story. We don't judge other traits on whether they are inate. We don't accept violence from XYY males (okay, that's been disproved -- but we wouldn't if it were a valid link). But, what the heck, if it works to help some people come around to accepting transsexuality, I'll swallow my pride. I'll just tell myself it helps people see it in a different light, and maybe that's what they need.