The open command in Terminal will, when given a vnc://1.2.3.4 (or vnc://hostname) protocol argument, open the Screen Sharing app and connect to IP address 1.2.3.4 (or hostname).
[robg adds: I was certain we'd covered this tip somewhere before, but all I can find is a brief mention in this comment to this hint about dict:// URLs.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2010031703564770