Like most airlines, United has, and strongly encourages the use of, an online check-in tool so that passengers can check-in for their flight before leaving home. This is seen as a win-win situation for everyone. United gets the user to do the manual labor of checking in and paying for the paper stock for printing the boarding pass while the user gets to avoid check-in lines at the airport.
I am a big fan of using this and typically check in near the limit of 24 hours before departure. I check in this early in part so that I don't forget to check-in in the mad rush out of the house on the day of my flight and in part so I can check to see if there's a better seat available at check-in.
However, this has led to one problem. On several recent flights, I was upgraded sometime between my early check-in and my departure for the airport. Because I was already checked-in in coach, I was unable to select a seat in the first class section. Theoretically I should be able to un-check-in and then re-check-in, or I should be able to get the customer service people to do the same for me, but neither worked and I had to wait till I got to the airport and the gate agent opened the flight at the gate (even the Red Carpet Club agents were unable to help me).
Moral of the story: If you're on the upgrade list, don't check in till you're close to leaving for the airport.
Update (10/21/07): Not learning from my own mistakes, I checked in around 11:30 PM the night before a flight to Tokyo as it appeared that there was no chance that it would clear before leaving for the airport in the morning (it was a 12:41PM departure). Of course, I was wrong and the upgrade cleared at 5:03 AM. But, since I was already checked in, I couldn't select seats in business class. Checking the site (by the usual trick of trying to purchase a business class ticket) showed that there were still 4 seats open including a coveted aisle seat (8D). By the time I got to the airport and checked in, the only seat left was 13E (a middle seat). Hopefully you'll learn from my mistakes better than I do.