They explicitly said so when I interviewed in March. However, not having had one, I don't have first hand knowledge of them pursuing the issue with anybody. They also didn't come in to my ground school class and make any request for resignation letters.
However, the current request by the Delta MEC for Comair to give preferential hiring to furloughed Delta guys and to allow them to keep their Delta seniority number is partially based on changing the current policy, which is that Comair expects you to resign your seniority number with another airline.
B4 all the furloughed guys get in a tizzy however, I don't have any feelings on the matter one way or the other, just answering the question. As long as everyone who is hired after me, is junior to me, all will be right with the world.