(1/?)
I envision actors having public keys as their IDs, possibly with some system for updates/revocations. When actor references are passed, they're sent together with the current (or last known) network location of the actor. Each server keeps a directory mapping IDs to locations. Actors can transparently move between instances, by sending a Move notification about their new location. Anyone who misses the notification can query friends/others who did get it.
(1/?)