I had a problem with document libraries where users were opening documents they posted and they were automatically opening in read-only mode.  In some cases, with contribute rights to the doc library they were still able to modify the document and in addition, would not receive the prompt to enter versioning comments.  A fix I found was to install at least one product in the Office 2007 suite.  So far so good, the user is still running primarily Office 2003 products and with one Office 2007 product installed they now receive the prompt to select read-only or edit mode.  In addition, upon closing the document they also receive the prompt to enter versioning comments.  Version functionality in MOSS 2007 is pretty slick when it works properly!