Well, honestly, transsexualism not being a directly genetic problem (or if it is it hasn't been shown), I think that if there were a prenatal diagnosis of transsexualism, in the event that it actually is related to hormone levels during the development of the brain as it's currently believed to be, merely correcting those levels would prevent the disorder from appearing, creating a normal and happy child.
Some people think that ->-bleeped-<- is a good thing, but I definitely disagree. The most basic concept of being transgendered is to not feel like you belong in your body as it is; in other words, to feel unhappiness. Many argue that the unhappiness of a trans person comes from the reaction of society. This can't possibly be true. If hormones and surgery were free, fast and painfree; if being a transsexual was absolutely no big deal; if everyone was 100 % supportive... would I stop feeling bad about myself; stop wishing that I were born a girl? Of course not.
So if trans = sadness, considering that sadness is an incontestably bad thing, why would we find it to be a good thing? And why would we object to the prevention of it?
Considering the price of hormones and the (I think) relatively short time frame during which the foetus develops its brain's gender, I doubt that such a treatment would be more expensive than abortion. So I don't think people would just flush away transsexuals-to-be.