NW was already planning on parking a good number of them prior to the merger announcement and/or oil price going through the roof...
As far as parking them, the question is: At what point do fuel prices outweigh the fact that they're paid for? As it is, NW can park them or put them in service almost at will without eating a lease payment.
Cajun Flyer--
Have you been on a NW DC9 lately? They were all overhauled not too long ago. The interiors are actually quite nice. As for reliability, I don't have the current numbers here at home, but at one point the DC9 fleet had the highest dispatch reliability in the fleet.