This was actually covered in another thread as well (about the 742).

NW will run these subs when there are major flight disruptions like what happened at SEA this weekend. They subbed in two 747-200's yesterday and one A333 today (which did go onto DTW as you mentioned). I'm not sure if there were any substitutions/additions for PDX; I didn't look them up.

NW also used to run the A333 as a scheduled flight between SEA and MSP last year for awhile. They will sometimes do that when a new plane is entering rotation and needs to be flown domestically for awhile.

-RM