I was booked on UA371 ORD - PDX on Saturday. When I arrived in ORD from PHL, I discoverd that the scheduled 4:45PM departure had been changed to 6PM. I also noticed that there was a 3:15 flight to PDX that was on time. So, I went to the gate and asked the GA if I could go on standby for that flight. After making a phone call she told me that no, since I had checked luggage, I had to travel with my luggage. I didn't argue with her and went and found a nice barstool to wait things out. Well, the flight boarded about 6:10, and then things went downhill. I was in seat 6A (A320), so I could see crew on a ladder looking at the engine. They brought a piece of bent metal from the engine area onto the plane to show the captain, and sure enough, about 15 minutes later the captain comes on the PA to announce that there is a mechanical issue and they were trying to decide whether to repair or abort. Finally around 7:00, we were told to leave the plane and another plane would arrive for us at about 8PM. Well, I finally arrived in PDX at 11:30PM, and due to my great seat was one of the first off the plane. When I arrived in baggage claim, I discovered that my luggage was already there! It was off to the side in a roped off area with a bunch of other bags. the only explanation I can see is that they put my luggage on that earlier flight! I'm a lowly 2P, what do you more experianced flyers make of this - should I have argued with the GA to try and get on that earlier flight, especially since it turns out the the "you must travel with your luggage" would have worked out anyway! It's frustrating to think that I could have been in PDX five hours earlier than my eventual arrival time.