On Sun, May 04, 2008 at 04:21:41PM -0400, Yang Zhang wrote:
> $ ssh hv sleep 30 &
> [1] 22468
> $
> $
> $
> [1]+ Stopped ssh hv sleep 30

ssh(1) says:

-f Requests ssh to go to background just before command execution.
This is useful if ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This
implies -n. The recommended way to start X11 programs at a
remote site is with something like ssh -f host xterm.

-n Redirects stdin from /dev/null (actually, prevents reading from
stdin). This must be used when ssh is run in the background.

> Furthermore, I can't kill this process using sigint, sigterm, sigquit, etc.:

If a process has been stopped, it won't respond to any other signals
until it's told to continue.