Save this AppleScript to “~/Library/Scripts/iChat/Growl.applescript”. via http://amccloud.com/lion-ichat-growl-notifications-50108

iChatGrowl.applescript

using terms from application "iChat"
	on message received message from theBuddy for textChat
		set whoDidIt to full name of theBuddy
		set buddyIcon to image of theBuddy
		tell application "System Events"
			set frontApp to name of first application process whose frontmost is true
		end tell
		tell application frontApp
			set window_name to name of front window
		end tell
		if whoDidIt is not in window_name then
			tell application "GrowlHelperApp" -- ** the daemon that is behind the scenes
				-- Make a list of all the notification types that this script will ever send:
				set the allNotificationsList to {"IM Received"}
				-- Make a list of the notifications that will be enabled by default.
				set the enabledNotificationsList to {"IM Received"}
				-- Register our script with growl.
				register as application "iChat Growl AppleScript" all notifications allNotificationsList default notifications enabledNotificationsList icon of application "iChat"
				if buddyIcon is equal to missing value then
					notify with name "IM Received" title whoDidIt description message application name "iChat Growl AppleScript" sticky no
				else
					notify with name "IM Received" title whoDidIt description message application name "iChat Growl AppleScript" sticky no image buddyIcon
				end if
			end tell
		end if
	end message received
end using terms from