Hi there,
 
We've heard that there has been some questions on which versions of Microsoft Office Communicator are supported with Outlook 2010.  The supported versions are Office Communicator 2007 and later versions (including OC 2007 R2 and Lync 2010). Office Communicator 2005 is not supported with Outlook 2010.  This means that there is no integration between Office Communicator 2005 and Outlook 2010.  You can run them on the same computer as stand-alone applications but you won’t get any of the integration benefits…like presence awareness on contacts, the ability to start an instant message from Outlook, and so on.  If you have Office Communicator 2005 in your environment and are planning to deploy Outlook 2010, then consider migrating to Lync 2010.  
 
What is Office Communicator? Office Communicator is the Microsoft Office Communications Server client application for real-time communication.  It provides  rich presence awareness, software-powered voice capabilities, instant messaging (IM), multi-party audio, video and desktop sharing, and integration with Microsoft Office 2007 and Microsoft Office 2010 (with OCS 2007 and later versions).   For more information about OCS 2007, see Microsoft Office Communications Server  on TechNet.
 
What is Lync 2010? Lync 2010 is the next generation of Office Communicator.   If you haven't already, check out the product info and see it in action: The Next Generational of Unified Communications - Microsoft Lync Server 2010.
 
Hope that helps!