Impact of gravity on bone structure Some suggest that if a human was born and raised in a gravity-less environment their bone structure/strength would be different that for children born on earth. If this is the case do you think this change would be gradual (over several generations) or would it be immediate? Also would these changes be permanent in the DNA (genetic mutation?). If the said human, born in a gravity-less environment comes to earth, surely they would have problems dealing with the effect of gravity but would they be able to acclimatize gradually ?
 A: I don't think that physical differences in skeletal structure would be apparent immediately because while yes there is less force acting on your growing bones, a large portion of that growth occurs while we are sleeping and horizontal. Our bones would have less weight to support but their overall structure is defined by our genes not our environment. The immediate effects would be in the muscles. Our muscles would have less force acting on them so they would not develop in the same way they do here on earth. 
The muscle changes would not be permanent because with just more exercise the equivalent amount of work on earth can be accomplished, however just as dagorym said the gradual long-term affects on the skeletal system would be genetic and permanent. People with aforementioned brittle bone disorders would survive much more readily and similar disorders that produce elongated limbs would not be fatal and the genes would be passed on. Over time this would lead to a species of human that was much thinner and taller but about the same mass.
Because of these changes they would have a tough time under increased gravity. Sure, their muscles would strengthen but the underlying skeletal differences would be unavoidable. No amount of exercise can increase your bone density so they could not immediately compensate. 
A: I suspect that some direct physical differences would begin to show immediately.  Children born an raised in that environment (whether gravity free or just lower gravity) would not have the same forces acting on them.  Their bones could grow longer faster as there would be less force for the bones to support.
Permenant DNA changes would be much more gradual and probably come about due to lack of selection against genes such as those that cause thing like 'brittle bone' disorders.  (My boys have a friend that suffers from this.  He has to be be very careful as even a simple fall can result in broken bones.  He'd do much better in a low gravity environment).
However, I don't thing changes could be that drastic, at least at first, the human bone structure still has to be able to support the mass of the body.  Just because there is no gravity, you still will have forces working on you when you want to move (think pushing off and landing as you move across the room) and your bones will have to support this.
For someone raised in a low gravity environment, coming back to Earth would be a bit of a problem.  In addition to a weaker bone structure, which may or may not require that the person be confined to a wheelchair for support, you would also have the issue of the cardio vascular system would not be used to working on a 1 g planet and may have a hard time pumping the blood around the body as it has never had to work so hard.  They could possibly acclimate but it would not be very fun.
