The trailer does seem to give away a few hints. More obviously that the courier has been to the Divide before since he knows the way. But more telling is the part where Ulysses tells the courier to bring whatever flag you're carrying now. Makes it sound as if the Courier and Ulysses were fighting for the same cause at one point in time (What the United States used to be?) But for some reason the courier swapped allegiances. I would also venture to guess that Ulysses had not been to the divide before given his interest and needing to learn about it at the Big MT. My thought is that Ulysses refused the job because he wanted to the Courier to see for him/herself that fighting for the factions on the strip aren't worth fighting for. Then again who knows it's 330 in the morning here..

http://www.gamesas.com/index.php?/topic/1226227-ulyseeses-the-couriers-relationship/