Try to type ichat: in your browser and press "enter". It opens Ichat. Works with IE, Icab, and maybe others.
[Editor's note: This won't work in Mozilla without a bit of work ... using the same technique as described in this hint, add a new helper for "ichat" and point it at the iChat application and select the "Use current application" checkbox. Quit and restart Mozilla and you can now use the same trick.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020923050636171