On Thursday I flew FRA-ORD-BUF in coach. My Itineraries page had shown me as wait listed for an upgrade on the ORD-BUF segment. When I got to ORD I cleared customs and went to the gate and saw that I was #2 on the wait list for the upgrade, however neither one of us got it. No big problem, it's a short flight.
However, when I got home and had access to email again, I saw that I had received an email stating that "your upgrade has been confirmed", sent a few hours before my ORD-BUF flight.
Why was the upgrade not there when I got to my gate? Should I have checked in at a UA desk when I first landed in ORD? Do upgrades disappear if you don't confirm them?
Thanks!

Off-topic, I found it very strange that on Thursday, the first day UA was flying from Europe after the volcano mess, the plane was only 3/4 full. My parents flying LH FRA-LAX said there were lots of empty seats on their flight. Any ideas why???

DanK