I have 35,000 FF miles (not enough for a trip cause you always need 50,000) and thought I'd check on a mileage upgrade roundtrip non-stop from DTW-LAS. NWA.com shows a mileage upgrade to first class of $2111.40 + 10,000 miles. I then checked the regular first class fare for the same flights which showed a price of $506.41. That's almost 4 times the price just to use miles.
It's obvious that NW does not want anyone to redeem miles.