Check out these instructions on Bigblueball.com on how to get google talk configured to be a universal messaging client which will provide access to friends on all the primary messaging networks (AIM, Yahoo, MSN, ICQ). I’m hoping Adium on the mac can ultimately avail itself of the same configuration, but until it can I’m sorely tempted to switch over to google talk on the pc so that I get access to their excellent VOIP setup.