I'm so glad I have a tivo and could record the episode. I just saw the episode (sans commercials of course I guess I'm a thief according to Ted Turner)
The letter was pretty good. I don't remember the exact wording of it, but it started off with Dr. Greene saying he was sitting on a beach in Hawaii, enjoying the margaritas. He talked about how his kids were looking for seashells on the beach. He said that aside from being with his family, being in the ER was his favorite place to be. He mentioned that many times he considered going into a private practice but in the end the ER was the best place to be. Mark said that he had to leave his last day the way he did (not saying goodbye, just dissappearing) because it was the only way he could. There were things that he wanted to say to everyone, but he wasn't going to list them in the letter. Most of the things he would say the people in question already knew. I might be missing a point or two, but thats most of marks letter. Carter was reading it to all the staff and they werehaving a good time. Then Carter flipped to the last page and found out that it was Dr. Cordey that sent the letter, and apparently Mark had died that morning at like 7:00am.