UA vs NW - same thing happened to me when flying on a mixed NW/CO itinerary in January 05. I got upgraded on all segments, but could not print my boarding pass at home (thought I was SSSSS'ed). I asked for help at the airport, got the usual "why don't you check in yourself with this nifty machine we have here?," explained my problem, and finally got my tickets after the agent pressed a whole bunch of keystrokes. Might be some sort of a bug when flying CO and NW on the same ticket...