To make it easier for those needing to work with older file formats of DWG files you have been able to automatically save to older versions. Now the Wblock command also respects this setting. It's also possible to manually specify the file type in Wblock (Write Block) using the browse for file name and path dialog box. The setting is available at Tools>Options>Open and Save>Save as.
When reference editing a drawing it keeps the version of the xreffed drawing. You will get this dialog box message if you Refedit (reference edit) something other than a 2007 DWG: "The Xref selected references a previous drawing file format. Edits saved back to the Xref file will be in the release format of the referenced drawing file."
These are good news for companies that might not be able to upgrade all users to AutoCAD 2007 at the same time and having to use older versions during a transition time.