I wouldn't count on being able to transfer your miles to another airline in the (unlikely) event that NW goes under, but any airline excecutive with half a brain would jump at the opportunity to take over NW's elite flyers. You'd probably just trade your card for that of another airline.

As for miles, there are just too many miles outstanding, and they begin to represent a real danger to airlines' viability, and I can imagine that this would be a great opportunity to dump some of that liability.