I really like Growl notifications and also iChat. Before Lion I was using Chax patch that among other things allowed you to have Growl notifications in iChat. However, after Lion upgrade, the patch did not work anymore. Actually, many of the Chax features were built in the new iChat 6 from Lion.
So, I was researching in the net for a script that would allow me to use Growl notifications with iChat and I actually found this from Scripting OS X. However the script was for older versions of Growl, not the latest 1.3. So with my tiny knowledge of apple script, I managed to modify it to work with Growl 1.3. Also, I modified it the way that I only get Growl notifications if iChat is not the active app. I don't want to receive Growl notifications of messages that I am actually reading if the chat window is the active one.
So if you are interested, please feel free to download it from here.
UPDATE: if you do not want message text to show, please try this other script from here.
In order to activate the script, copy it into the Applescript Editor and save it as "Growl iChat" as a script file. Make sure you save or place the file in this directory (create it if it does not exist):
To configure it, open iChat, go to Preferences -> Alerts and select the Event “Message Received”, check “Run Applescript” and choose “Growl iChat.scpt”. Actually, the script also reacts to “Buddy Becomes Available,” “Buddy Becomes Unavailable” and “File Transfer Completed.”