Starting today, actually, Delta has changed the cancellation policy for all award tickets regardless of when the ticket was purchased. Going forward all award tickets must be cancelled at least 72 hours in advance of your flight to get the miles redeposited back into your account (free for Platinum and Diamond elite members, $150 per ticket for everyone else). Additionally, there will be no changes allowed to your flight within 72 hours of the flight as well. In the past, no advance notice was needed, which gave people the opportunity to change their ticket last minute or not even show up for the flight if something came up. Additionally, with Delta many of their “low award” availability appears within a few days of the flight departure, so travelers were able to change their ticket to use less miles, when available. This will no longer be the case – if you cancel within 72 hours you lose the miles, you are not even able to pay a penalty.
Of course, in their new policy they do explain that if there are extenuating circumstances (I am assuming a doctors note, a death in the family, etc.), the customer service desk will be able to redeposit the miles (on a case by case basis).
Moral of the story – make sure to not wait till the last minute to change or cancel a Delta flight booked with miles.