Generally if you have diabetes then you have it for life though there can be a few exceptions.
1) Most cases of gestational diabetes resolve themselves though the mothers are at increased risk of developing type 2 later on.
2) Drug induced diabetes ie you can be on so many drugs that increase your blood sugar that your body can't produce the insulin to counteract it. Once the drugs stop then the diabetes goes - but again you are at increased risk of developing type 2 later in life.
3) Certain medical conditions can cause diabetes to develop such as an adrenal tumour - the tumour produces so much adrenaline that the body can't counteract with increased insulin. Once the tumour is removed the diabetes goes away.
4)A large weight loss can result in diabetes disappearing, as basically the body has been moved into a state were it is able to regulate itself, carrying more weight generally increases the amount of insulin that you need. In this case though there is obviously a high chance of the diabetes reoccurring if the weight goes back on.