Also, contact your CC company.
I booked some VERY expensive flights to Europe yesterday. The card I have is pretty generous so I am not worried, but as a general rule you have 60 days after the purchase for full protection. I figure the union will call the vote before 60 days is up.... so we will know.
(I also think that DL will not cease flying unless the pilots lose all of their common sense. While the union seems to believe that they could all get a job tomorrow making more money with better benefits I really think the pilots who have families to support will vote to keep thier current job)
If DL gets that close to "receivership" then someone will step up and take them over (NW is an option or vice versa there). In that case the "new" airline will honor your tickets.