I just tried checking in with nwa.com for my 10:35 flight out of SEA tomorrow (22 hours from now). Here's the message I got:

The nwa.com Check-In service is only available less than 30 hours and more than 90 minutes prior to flight departure.

Please see a Northwest agent at the airport for assistance if you are within 90 minutes of departure or try again later if you are more than 30 hours from departure.
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I'll try again before I leave for Seatte this afternoon, but I'm disappointed this service doesn't work as advertised. If that still doesn't work, I'll just use the terminal at the airport tomorrw, but I really wanted my 2000 bonus miles.