If you're running GNOME, chances are ssh-agent is already running when you log in. The above command prompts you for your SSH password then adds your SSH key (at least, that's where I keep my GNOME SSH key) to the running agent. And that is the last time you'll have to enter that password until your next log in.
This was very exciting to me...
UPDATE: I guess seahorse (slated for inclusion in GNOME 2.18) does the same stuff, but graphically.