Script to run ssh-agent, and ssh-add on login

bash_profile.sh

# Add to ~/.bash_profile

# Start/Reuse SSH Agent - restart or re-use an existing agent
SSH_AGENT_CACHE=/tmp/ssh_agent_eval_`whoami`
if [ -s "${SSH_AGENT_CACHE}" ]
then
  echo "Reusing existing ssh-agent"
  eval `cat "${SSH_AGENT_CACHE}"`

  # Check that agent still exists
  kill -0 "${SSH_AGENT_PID}" 2>-

  if [ $? -eq 1 ]
    then
    echo "ssh-agent pid ${SSH_AGENT_PID} no longer running"
    # Looks like the SSH-Agent has died, it'll be restarted below
    rm -f "${SSH_AGENT_CACHE}"
  fi
fi

if [ ! -f "${SSH_AGENT_CACHE}" ]
then
  echo "Starting new ssh-agent"
  touch "${SSH_AGENT_CACHE}"
  chmod 600 "${SSH_AGENT_CACHE}"
  ssh-agent >> "${SSH_AGENT_CACHE}"
  chmod 400 "${SSH_AGENT_CACHE}"
  eval `cat "${SSH_AGENT_CACHE}"`

  # add your own key
  ssh-add ~/.ssh/some_key_file
fi 

For anyone stumbling upon this now, it's no longer necessary. See http://superuser.com/a/269570/393696