If you add certain columns to a content type they will map back and forth to built-in properties of any Word document. This works, for example, with title, subject, and category. These columns are all in SharePoint 2010 as primary document columns. This is documented on MSDN.
The interesting thing is that the document information panel from Word and SharePoint are synchronized for these properties. I.e. entering text in one panel will update the field in the other panel.
There is also a document property in Word called 'Abstract'. Can this be mapped to a site column in SharePoint as well?