You should be OK. Just keep watching your particular flight as there may be schedule changes and reduced flight frequency due to NW filing chapter 11 (bankrupcy protection)
Air Canada was in this situation last year. All the majors in the US (United, American, USAirways, now Delta) have filed since 9/11. USAirways filed twice and they are still flying.
It just means they have to reorganize and reduce costs. Since NW is such a big carrier out of the north east - Detroit in particular I doubt if they would cease to exist right now. They'll probably just come through chapter 11 a bit smaller.
The high cost of fuel is one of the major factors for NW and Delta filing chapter 11 in the same week.