Our WSS3 document libraries no longer prompt to check out. I've confirmed check out is required for editing, and I've tried a new library. We're getting the same results. I also check the authentication provider and ensured client integration is enabled. This has been working great for quite some time. Users are using Office 2003. Could there have been a Windows Server update which disabled this for the old Office version?
When attempting to check out the file within SharePoint, using it's resident checkout functionality I get an error
The file "MyFile.xls" is not checked out.