I haven't read the thread so I'm sure I'm repeating stuff already posted, but here goes!
Bankruptcy is very differant from an airline going out of business. Bankruptcy means that the airline is protected from having to pay their creditors until they get the company back on track.
Going out of business means they cease operation, meaning they actually stop flying and have a "going out of business sale" on office furniture!
What you are worried about is them actually going out of business. If that happened (and it most likely won't) here is what will probably go down:
The other airlines (United, American, USAirways, etc.) will honor your Delta ticket, at little or no cost, for any of their seats that are still empty. So if United has a half full flight, they'll be happy to take your ticket on Delta and get you down to Florida. But, if United has no seats, then you're out of luck, since of course they won't bump their own passengers to accomadate you. At this point, since Delta is belly-up, they won't help you OR refund your money. You'd have to sue them for it, and you'll be behind jet fuel companies and jet leasing companies to get your money, they get first dibs! Best you can hope for is that your credit card company reverses the charge.
Anyway, you don't have anything to worry about. United has been in bankruptcy for ages, and they're still flying! Don't worry about it and enjoy your trip!