Our SharePoint database server ran out of disk space. In addition to causing the obvious problems, we were also seeing this message in the SharePoint front end server logs:
Failure trying to synch web application ddbd3051-0aa4-4db5-8197-e450a718dede, ContentDB 7393dd21-a0fe-42f6-8d6b-b87c8ce31496 Exception message was System.Data.SqlClient.SqlException: Lock request time out period exceeded.
Turns out that the cache was out of sync, which was resolved by following this kb article:
http://support.microsoft.com/kb/939308