Below is the link to the NW policy on carry-on luggage from their website. I've taken 4 roundtrips/16 segments on Northwest and from the lack of overhead space for my bags each time I would agree with the liberal comment. Since your trip is international the website does say you only get to have one carry-on while other US carriers allow two. I imagine this is because of the close relationship with KLM. By the way, welcome to Flyertalk.
http://www.nwa.com/services/shipping...q.shtml#intern