Both of my boys are in the GSP program at their school. My oldest was recommended to be tested by his K teacher because he was an overacheiver and completely bored with school at 5. He tested high enough to enter the program and placed in the program. He has the ability to get straight a's but doesn't. He is not "Doogie Howser" gifted.
My youngest was recommended to be tested for learning disabilites and it was determined that he was gifted instead. He gets poor grades and refuses to write anything. Two different courses to get to the same result. He is also not "Doogie Howser" gifted.
The gsp program that my kids are in is not just about getting out of class once a week and going on extra field trips, as someone mentioned. The kids I have encounteed in the past 6 years (and more to come) think differently and are in need of the extra stimulation they get: problem solving and creative thinking exercises that they receive once a week. Their brains are wired just a bit differently. The field trips they go on, that I have had the pleasure of being a part of, have been in conjuction with competitions, Odyssey of the Mind and Future Problem Solvers, that they have been preparing for for several months. They have to keep their grades at a certain level in order to participate in these activities.
I have not read the other threads many people are referring to, nor do I intend on "flaming" anyone. I do think people have a tendency to overstate their kids abilities to make themselves feel better. I have encountered them at OM and FPS. Some are loons.
However, most of the parents I have dealt with on a regular basis (the parents of the kids that are in my guys school) have their heads on their shoulders and are realistic about their kids abilities.
I was never in a gifted program, but I got all a's in school and did very well in college. I also do not think my kids are any better than anyone elses, nor are they my little "snowflakes". I am pretty realistic about them, and about their abilities and inabilities.
EDIT: as to the above post about the child prodigy, I know my kids are not this! I wish they were though, they would be making me money

And with all the gifted kids I have encountered over the past 6 years, I have never met a child prodigy, those are truly far and few between.