up an xterm. If you are not running X, will use a bit of applescript
to fire up a Terminal window.

Note that you need to be running an ssh-agent, started when you log
in. One way to do that is http://www.sshkeychain.org. There are
probably other ways as well, but not sure what they are. For
sshkeychain (which I installed and seems to work okay) here are the
instructions:

    * Copy SSHKeychain (SSHKeychain.app) to your Applications folder.
    * Run SSHKeychain. This should open a dock item and a statusbar item.
    * Click either the Statusbar Item, the Dock Item, or MainMenu and
      open the Preferences.
    * Open the Environment tab.
    * Enable "Manage global environment variables". This will make
      SSHKeychain available for other applications.
    * Open the keys tab and see if any of your keys are missing
        (~/.ssh/id_dsa and ~/.ssh/identity are default).
    * Add SSHKeychain to your Login Items in the System Preferences.
    * Re-login to make the global variables work.
    * Start up SSHKeychain, and you're set.