Ever since the publication of Janice Raymond's The Transsexual Empire: the making of a she-male, there has been resistance to the inclusion of transsexual women into "women-only" type spaces. The assumption seems to be in those quarters, that if you're not female, then you're not a woman. This stance is justified by various ways, and phrased differently, but that is what it means. I think the basic issue here is one of definitions, assumptions, and real intent.
What is a man, what is a woman? I have my own definitions (as listed as links in the previous sentence) but others have different definitions. The problem is that female is mixed with woman -- they are not the same. An all-female space is something different from an all-woman space. But even a definition such as this would not be iron-clad, for it would allow entry to transsexual men, and that, I think, is something not quite wanted.
I think that there is an underlying assumption on wymmin's part, that transsexual women are:
a) always detectable, and
b) still behave like men.
But basing inclusion upon "passing" is not a wise choice, because some non-transgendered women don't "pass" either! And many women have masculine behaviour traits. The mistake it makes is to assume that transsexual women have only changed our bodies. I believe anyone who only changes their body, and nothing else, isn't going to want to be included in such space anyway, and isn't going to be successful in their transition.
One assumption is that transgendered women have different histories from other women. The underlying belief here is that all women will have certain histories in common, and I think this is untrue. The commonalties vary from culture to culture, and from person to person.
Finally, as I see it, the real intent is one of fear and safety. If transsexual women are seen as a means by the patriarchy to infiltrate womens' movements (and we are by some people, very "noble" of us isn't it?) then we appear as a threat. You cannot demand entry to such a group, if this is how you are perceived, because that only reinforces such a view. But not all non-transgendered women feel this way. And if the desire is to create a safe and nurturing space for all women, it seems ironic and unfair that one segment of women (transsexual) be excluded.
You might also want to read the essay I did in reply to an article that appeared at the 1998 NOWSA conference.