My son misses the cutoff by almost 3 months, but we have decided to send him early. He's 4 and reading at a 4th-5th grade level, and doing 1st grade level math and handwriting. He has been in a preschool class full of kids who *do* meet the kindergarten cut-off, for 2 years and is one of the more socially/behaviorally mature kids in the class (according to his teacher). We really feel he is ready for kindergarten (or even 1st grade, but we don't want him to miss K). The private school we are sending him to does testing and interviews of all applicants and said we could apply despite the cut-off (and the have taken kids who missed it by <1 month before). He did well and was accepted. I feel secure knowing that he tested into the grade, and that his preschool teacher (who has had him for 2 years) thinks he should go to kindergarten. Also, he has been complaining of boredom at preschool.
After a year of private K we will have to decide whether to try to switch him to public (which would be a fight against the 1st grade cut-off) or keep him at the private school.
I really think that outside assessments are needed before choosing a grade skip. (or I could just say "yeah" to everything cyclingmom just wrote). Being a little ahead and one of the oldest (within the cut-off) usually goes great for kids. Being several years advanced in kindergarten is less likely to work out, IMO.
My 7 yr. old is one of the oldest in his class and I love it. He is super popular, excels at sports, and has better behavior/social skills than many of his classmates. We're moving so he won't be one of the oldest anymore (cutoff is much earlier) but I think the confidence will stay with him.