Up until now, when remailers have gone down it has not been in a circumstance in which escrow would help much. Generally they are taken down voluntarily or at the request of someone in authority (the owner of the computer they are running on, in many cases!). In these situations there would not be a problem in transferring the key to someone else. There have been a couple of times, particularly early in a given remailer's life, where people have clumsily deleted their remailer's key and had to create a new one. In such a circumstance a secure backup capability would be useful. But you don't really need the kinds of recovery that Matt's idea provides. Conceivably a remailer operator could be dragged off in chains, but it doesn't seem like a very probable scenario. Even then more traditional secret sharing based distributed backups would seem like a better fit than "net escrow". The main distinguishing feature of the latter is that society as a whole can choose to release a key without the owner's approval, but not any lesser group. Hal