When adding Office 2003 documents to MOSS custom document properties in Office 2003 are added as SharePoint properties in a content class. Setting custom documents properties in Office 2003 is easy and well known.
When uploading Office 2007 documents to MOSS with custom documents properties set in the same manner as in Office 2003 documents, MOSS properties are not set.
I found this blog which points out a solution and it works fine if you're working with Office 2007 templates. However, if your template is a Office 2003 DOT-file and saves the document as an Office 2007 it doesn't work anyway.
Update: I found the following msdn blog which is worth reading.
