I have tickets on Song for 4/16 and 4/30. I called 1-800-FLYSONG. They told me that if the pilots strike, Delta will not attempt to continue service (presumably by hiring non-union pilots). In that event, ticket fares would be refunded to all affected ticketholders.
If you are in the middle of your itinerary (that is, if you've already flown to your destination and you're going to have to fly home), Delta MAY make arrangements to get you onto another airline to complete your itinerary. However, if you have not yet taken the first leg of your flight, Delta will make no attempt to re-book you on another airline.
So the good news is (if you believe the customer service people at Delta) that if your flight is cancelled due to the strike, you'll get your airfare back and hopefully you can re-book on another airline.
Hopefully, they work something out and there's no strike. Maybe another extension for 3 more months. That would be nice.