A user of Access 2007 without SP1 will get an error when trying to open an ACCDE or MDE made using Access 2007 SP1:
This database is in an unrecognized format. The database may have been created with a later version of Microsoft Office Access than the one you are using. Upgrade your version of Microsoft Office Access to the current one, and then open this database.The only fix is to install SP1.
Details: http://support.microsoft.com/default.aspx/kb/946205
