On the off chance you're trying to get Gaim instant message windows to have this behavior, there are hacks you can use. One of the plugins or settings for Gaim will let you change the window title of a window where a new message is pending. You can set it to add some strange combination of characters, like [**] to any window with a pending new message. Then set your window manager to do something to windows with those characters in the title.

I doubt you can unhide kicker, but you may be able to do something to the window itself, like raise it or something. If you're able to capture the event you want, and if you have the option to script an action, you can do nearly anything. Stick an XOSD message on the screen saying "NEW MESSAGE", for example.

You may also want to check what DCOP commands are available for kicker and for other apps, if you're using KDE. You can do a lot of screwy stuff from the command line in KDE via DCOP.