When using a SharePoint 2010 Web Application for your portal/mysites with Claims Based authentication the Outlook social connector cannot connect to it!
UPDATE: SP2010 CU1 fixes this!
With Fiddler you will see that Outlook will try to connect to http://portal/_vti_bin/Authentication.asmx
But this returns a response with the word (ModeResult) “Forms”
and that’s why Outlook will show that it cannot connect!
There is a sort 0f solution for this, create a second webapplication, with a separate url, and connect it to the same User Profile Service application as your portal but configure it with windows authentication. Now users can use this second url in their Outlook Social Connector for Sharepoint.