Yes, it seems to be the rule that as per santo's URL that non-online purchases get the 24-hr rule, but online purchases get till midnight the next night to cancel as per this URL:

http://www.nwa.com/purchpolicy.html

"Northwest Airlines nwa.com Purchase Policy

Thank you for making your nwa.com Reservations' purchase. If you change your mind, our Customers First policy allows you to request a refund without penalty if you cancel your reservation by the end of the next day (midnight Central Time) by calling 1-800-225-2525.

If you don't cancel the reservation, however, you will not receive a refund unless you have purchased a fully refundable fare. Please note: Some special offers do not allow a 24 hold. "

Just for the record, this extra time has been invaluable to me as I am now an independent consultant, and when the client is in doubt about further work, it gives me an extra day or so to lock in a fare and if it's a no-go, to cancel the next night w/o penalty. If we're still in an uncertain condition, I can do the same thing the next night, thus assuring me of the lowest fare possible on that day. Of course, the fares keep rising usually, but it does avoid a 24-hr purchase which is often hideously expensive. Except sometimes, a LOWER fare becomes suddenly available. And, I can cancel the booked ticket and grab the new fare.

For me, it is a truly valuable benefit from NW, and I don't have to break or "bend" any rules.