Couldn't find any hits for this concerning 2010 but there were a few for 2007.
Sharepoint Designer 2010 Beta - SharePoint Server Beta Eterprise Edition - working fine one day. The next morning when as far as I know nothing had changed I could not open any sites.
Messages contained the following "Server is unable to open site "
I tried a few sites and got a few variations of the message which Unfortunately I didn't write them down as I was in too much of a rush to solve the problem :-( .
Text here is from memory - such as 'Content Type Could not be determined", Status = 'ok' and similar.
Following suggestions in WSS 3.0 Sharepoint Designer 2007 posts I recycled the app pools and all was well.