Vincom is right. Supposedly, "they" are working on automating the whole endorsement process, and some airlines within an alliance might already have the electronic capability. However, until then, you must have a paper ticket. I don't fully understand the endorsement process, because I've had receiving carriers accept a paper ticket that has not been endorsed and when no reservation on the receiving carrier had been made by the original carrier.

Back when YX shared the main terminal at DCA (as opposed to terminal A where they are now), I had a flight disruption with DL. DL was being a royal pain in the arse about the whole situation (or was it YX? I can't remember). The long and the short of it was that they could put me on an NW flight (long walk/bus at DCA from their terminal) but not the YX one. I got the endorsed ticket, walked right up to YX who was the next counter over, explained the situation, and got a boarding pass.

More recently, I missed a flight with NW out of IAD due to my own lack of planning. NW was jerking me around and wouldn't confirm me later seats or anything. CO would take the ticket, pending authorization from NW. I couldn't remember NW's response, but they wanted nothing to do with the situation. We went back and forth, and NW finally said they'd do it, but then claimed the computer wouldn't push the ticket over because it's a restricted fare. I walked back over to CO and asked them if they would take a paper ticket. (Yup.) I went to the WC (I wasn't a member, and all of NW's gates were closed) and asked the agent for a paper ticket. "I can't do that sir." Wanna bet? Nex thing you know, I got my paper ticket and my confirmed seat with CO.