Now that Yahoo and Microsoft Messengers have merged Mac users needn’t feel left out. Microsoft have announced version 6.0 of their Messenger service is now available for Mac. It features that interoperability with Yahoo! Messenger that we all apparently want.
A personal account works with the Windows Live Messenger service, whilst a corporate account uses the Microsoft Office Live Communications Server service. Microsoft say that the federation features on the Live Communications Server allow Messenger users to communicate with many other IM services including Yahoo IM, AIM, and iChat. Presumably the personal account only works with Yahoo.
And just for completeness, Yahoo have updated their Mac client to version 3.0.1 (beta, of course). Both are universal applications.