Just as a heads up, our SharePoint Online/Office 365 service hit problems earlier today with the sites suddenly going into read-only mode. Shaun has since found this:

SharePoint Online has encountered a service incident. Some customers may be able to access there SharePoint sites in Read Only Mode. SharePoint Online teams are continuing to work towards full service restoration. The next update will be on September 11 2013 at 14:30 UTC.
So if you are having issues then this will be why.