Delta will be refunding the $25 second-checked-bag fee to passengers who purchased a ticket prior to April 9 and flew on or after May 5. Passengers who fit this description have to fill out a rebate form, and will need the receipt from the flight (hope you kept it!).
Why, you might be wondering, is an airline voluntarily handing out rebates? According to the Wall Street Journal, Delta's fee took effect before it was formally announced on April 9. Understandably, passengers who fell into this time warp were unhappy to learn they had been charged a phantom fee, and they complained enough to convince the carrier to hand out refunds.
It's refreshing to see an airline doing the right thing for once, because I can just as easily imagine a cash-strapped carrier shrugging its shoulders and saying "tough luck." So if you were affected by the timing malfunction (and you kept your receipt), consider yourself one of the privileged few to get something back from a U.S. airline.