In my SharePoint 2010 environment I currently have the document library setting "Require documents to be checked out before they can be edited?" set to no. All of my users are on Office 2007 and Office 2010, but most are on 2007.
If one of my users opens a document (almost always an Excel or Word file) it usually opens in edit mode without a prompt because usually they aren't opening it directly from the document library. This isn't really a problem until someone else needs to be able to edit the document at which time the first user obviously has to close the document and reopen it after the other user has opened it and claimed editing rights.
If you open a document specifically in read-only there's usually a button in the information bar that allows you to take editing rights, but is there any way to back down from editing mode to read-only from within an Office document?