You can see the Gadget and get the code from here but the really neat thing about it, apart from being able to IM with video and photo previews (file transfer and voice isn't supported at this stage), is that you can even set the default Gadget colour, height and width to fit with your existing website template :)

Links:
GoogleTalk: http://www.google.com/talk/
GoogleTalk Gadget: http://www.google.com/ig/add?moduleurl=googletalk.xml
Google Gadgets for your WebPage: http://www.google.com/webmasters/gadgets.html
Adobe Flash Player: http://www.adobe.com/products/flashplayer/