Is male pregnancy (à la Arnold Schwarzenegger's movie "Junior" and the occasional hoax or Weekly World News article) possible?
Well, there's one kind of male pregnancy that happens pretty often, and that's "sympathetic male pregnancy", in which Dad gets symptoms like morning sickness at the same time Mom does. This is relatively unexciting, though, as it's still Mom who's carrying the baby.
Obviously men don't have the female hormones necessary to keep everything going, but presumably some sort of hormone replacement therapy could be managed. The main reason why it's not done is because it's so risky.
Since a man doesn't have a uterus, to make him pregnant you would have to open him up and insert the embryo surgically (by the way, how would you keep it from moving around before it implants?). In essence, you would be causing what in women is called an abdominal or extrauterine pregnancy, and which - according to statistics gathered from women - has anywhere between a 0.5% and 8% maternal mortality rate. Extrauterine pregnancy is considered very risky for the mother. (For comparison, the maternal mortality rate for a regular old-fashioned pregnancy is 1 in 1000, or 0.1%; another source gives the risk of abdominal pregnancy as being ninety times that of the regular kind). Furthermore, only about 5% of implanted abdominal pregnancies result in the birth (by caeserian section) of a viable baby.
The uterus is a specialized organ, and one of the things it does so well is deal with the large, 8-inch wound left when the placenta separates from the uterus after the child is born. Other organs that a baby might attach to, such as the liver or intestine, cannot take that damage as easily. Typically when the baby is removed in an abdominal pregnancy the surgeons are careful to leave the placenta in place, for just this reason. Risks include hemorrhage, infection, toxemia, anemia, disseminated intravascular coagulation, pulmonary embolus, and "the formation of a fistula between the amniotic sac and intestine caused by penetration of fetal bone" (see the last reference).
References:
"Junior": http://us.imdb.com/Title?0110216
on the (im)possibility of male pregnancy: http://urbanlegends.about.com/library/weekly/aa100400a.htm?once=true&terms=male+pregnancy
http://www.gentlebirth.org/archives/malepreg.html
sympathetic male pregnancy: http://pregnancytoday.com/reference/articles/malepg.htm
risks of regular pregnancy:
http://www1.iwon.com/home/parenting/parenting_article/0,18900,preg%7Cbaby%7C11435,00.html
a hoax male pregnancy: malepregnancy.com
more on abdominal pregnancy in females:
http://216.239.57.100/search?q=cache:eiSW44xh0rgJ:www.medscape.com/viewarticle/410628_3+risks+%22abdominal+pregnancy%22&hl=en&ie=UTF-8
http://www.hkam.org.hk/publications/hkmj/article_pdfs/hkm0012p425.pdf