short and simple for 10.5 (Leopard)
sudo sh -c "/bin/echo -n enabled > /Library/Preferences/com.apple.ScreenSharing.launchd"
slightly different for 10.6 (Snow Leopard)
sudo sh -c "/bin/echo -n enabled > /private/etc/ScreenSharing.launchd"
rm the file to disable it. launchd polls regularyly, so after you "enable" the service, it'll be loaded by the time you check for the open port with netstat -an