Confirmed. Just canceled a flight the day before departure and miles were redeposited without issue. Although the warnings did make me worry:

"You are attempting to cancel an Award Ticket that is within 72 hours of departure. Award Tickets must be canceled at least 72 hours prior to originating flight time to be eligible for mileage redeposit (exceptions may apply). Miles will not be redeposited".

However, once I clicked through to the next screen, it confirmed that the miles would be redeposited. Guess an exception did in fact apply? I'm a Diamond so maybe that makes a difference?