We recently came across an error when users tried to overlay their Exchange calendars on a SharePoint 2010 calendar. Basically, they would browse to the calendar.aspx page and select the Calendar tab in the ribbon. Next, they'd select the Calendar Overlay option and enter the OWA URL and Exchange Web Services URL.
When they hit OK and tried to view the calendar they got an error saying:
"Could not establish trust relationship for the SSL/TLS secure channel with authority"
The solution is to export the certificate you're using on your Exchange Outlook Web Access server to a .cer file and save it to somewhere you can access it from your local machine. Next, login to Central Admin and select Security - Manage trust - New
Here's an article with a little more explanation from Microsoft: http://support.microsoft.com/kb/982982