There is a world of difference between the social eugenics movemet of Galton et al, and the use of "positive eugenics" for screening of deleterious rare mutations for single factor diseases like CF, SCA, Tay-Sachs, etc. As has been stated ad nauseaum, there are some mutations that whilst causing inborn errors of metabolism, etc (the single factor diseases) they provide a boost to reproductive fitness in their heterozygous state within the environment in which they first arose, hence their persistence within said populations, SCA is the classic example cited.
So whilst we can all throw our arms up in the air about "we don't know what impact this will have on our evolution", well you've forgotten the most important aspect of evolution and mutations, it is primarily the environment in which they exist that dictates whether they are neutral/beneficial/advantagous. Most of these diseases no longer provide a reproductive advantage.
The application of pre-natal genetic diagnostic (PGD) techniques to screen embryos or potential partners for these mutations is primarily of a beneficial nature because they allow at risk couples to make informed decisions about bring a child into the world. They can then choose whether to proceed or forgo having children based on this knowledge. The important thing to note is that this is not compulsory, state-imposed, screening throughout the entire population, but targetted screening of at risk populations (I think Kevin linked to the sucess of the Tay-Sachs screening program among Ashkenazi Jews; it also applies to other populations with higher than normal prevalance of Tay-Sachs). The reason we don't see these screening programs implemented throughout all populations is that it would put a financial burden on country's healthcare systems that would not be warranted or justified by the number of disease-afflicted birth avoided (either by abortion or the parents deciding not to have children).
So we see the application of what I've termed "positive eugenics" (that's what medical genetics is btw) in multiple populations with succesful outcomes. The eugenics movement had no scientific basis and was entirely a reason to try and legitamise racism and the class divides that existed at the time of its conception (through a lack of understanding of human genetics). Nowadays the selection of subjective traits will always be tarred with eugenics, whilst strictly inaccurate, it is as equally morally distasteful (but not quite as abhorrent).
In conclusion (yes finally) the application of genetic screening programs in high-risk populations is beneficial and that implementing these programs outside of high-risk populations (and individual high-risk couples, i.e. those with a family history, especially applicable to diseases that display a dominant or co-dominant pattern of inheritance) would be entirely superflous, not to mention a drain of vital resources which could be used to treat other more prevalent diseases. Where as the application of screening programs to remove lower penetrance mutations would most likely have detrimental or unkown effects on population fitness by reducing variation.
The selective use of eugenics is beneficial. As with many things this little quote from Ben Goldacre applies:
Ben Goldacre wrote:I think you'll find it's a little bit more complicated than that