But what I've figured out is that OpenSSH 4.2 now allows "opportunistic multiplexing". If you have
ControlMaster autoand a
ControlPath ...set up in you
~/.ssh/config, the first time you connect to a host it creates and leaves open a (UNIX domain) socket that subsequent ssh's to the same host will use. This makes these connections come up much more quickly. (Here's more info.)
However, what I think I really want is something that actually leaves the master connection open for a few seconds after I log out. Then, stuff that does a series of sftp's or commands executed via ssh will be able to effectively pass the open connection along in between them, instead of opening up and tearing it down repeatedly like they currently do. (In particular, I'm thinking of arch/darcs over ssh.) I suppose that's what fsh is for, though...