Clearly you haven't studied genetics in much depth. No matter how much genetic testing you may do, it is still a crapshoot. There is no way to know EVERYTHING that you carry in your genes.

If the families of both you and your partner carried a recessive gene for a disease like cystic fibrosis and it had never before appeared in wither family, how would you know to look for it? (Yes, I know CF is commonly tested for now anyways, I just used it as an example)

There aren't tests that exist for every disease. Less than 2% of Down's syndrome cases are hereditary. There is no way to predict over 98% of Down's syndrome cases, though we do know that the risk increases as the woman gets older.

There is no 100%. I don't care if you are the finest genetecist(sp?!) in the world, you still cannot predict with ANY certainty that your child will be free from disease.