According to numerous accounts in the last hour -- AP here, Reuters here, WSJ here -- the current story from the Northwest flight crew that forgot to land in Minneapolis is that they were so absorbed in using laptops in the cockpit that they neglected to talk with air traffic controllers for more than an hour and didn't get around to descending.
I hate to say this about people with enough other problems already, but: that's simply impossible to believe.
Flying an airplane is different from driving a car, in that it doesn't take constant second-by-second attention to the mechanics of where you're going and how you're handling the controls. If you type out a text message while you're driving a car, you really are putting yourself and others in danger. But if you take a minute in an airplane to check a detail of the routing, or a weather report, or anything else that comes up, in most phases of flight nothing bad is going to happen. The plane will cruise along with its autopilot, and most of the time no other planes are anywhere nearby. (Obviously this doesn't apply in takeoff and landing, in busy airspace, etc.)
That's why some of the stories tut-tutting the pilots for breaking company rules by opening laptops during flight are beside the point. That's a for-form's-sake only rule whose violation may be"wrong" but is not intrinsically dangerous.