Add Northwest to the growing list of airlines charging passengers for checking a second bag. Delta, United, and US Airways recently changed their policies, and now Northwest has jumped on the bandwagon and will begin to charge passengers $25 each way for that second checked bag starting on May 5.
According to this Business Week article, the airline is also doubling its fee for overweight bags to $50. The new second-bag rule won't apply to first-class passengers, who will still get three bags for free.
My guess is that to avoid these fees, passengers will begin checking larger bags and trying to carry on more stuff. So get ready for more back problems and slower boarding.