Sexing of the embryo is usually done in sex linked genetic disorders in which only males or females are defected. If you can choose the sex of the offspring before implanting the embryo you can prevent defective births. However, it is possible to sex the embryo for just because the parents want it to be in a certain sex. Actually sexing to plan the family is done in some institutions. This is another controversial area. Should parents be let to choose the sex of their off springs?
It is also possible to choose one trait of the embryo i.e. you can choose your off spring to be blue-eyed. Although the technology is not yet well developed to diagnose multiple genes it will soon. And once more we encounter the slippery slope of germ line gene therapy.
Sources:
Harper, J.C. and Wells, D. ?Recent advances and future developments in PGD? Prenatal Diagnosis 19 (1999): 1193-1199
Ray, Pierre F. et.al. ?Successful preimplantation genetic diagnosis for sex linked Lesch-Nyhan syndrome using specific diagnosis? Prenatal Diagnosis 19 (1999): 1237-1241.
Sudbery, Peter. Human Molecular Genetics. Singapore: Addison Wesley Longman, 1998.
Lee, Thomas F. The Human Genome Project: Cracking the genetic code of life. 2nd Ed. New York: Plenum Press, 1992.