I'm a Kindergarten teacher and, interestingly enough, my district is going from half-day to full day next school year so we have been involved in this debate all year. I'm excited about the change because, as a teacher, it is so difficult to cram in everything that must be taught in half a day. Kindergarten really is the "new first grade" (not always such a great thing). I feel that what gets left out in half-day programs is the opportunity for children to explore, talk, develop socially, and play "productively". It is all just too rushed.
That being said, I do agree with all those who stated that a child who completes a half-day program will do just as well in the long run as a full-day child, as long as he/she has supportive, actively involved parents. Anyone who takes the time to ask opinions of others and researches and tries to make an informed decision fits into that category, so I have no doubt that the little person in question will do just fine either way. If all parents and home situations would be like that, the education problems in our country would be minor. What goes on at home is truly the key.