"Tags cannot set field values but fields can set tags" - http://infoqubeim.com/drupal5/index.php/node/4328

Could tags be somehow enabled to set field:value assignments?  In that way we could have the advantages both of tags (e.g. inheritance) and fields (e.g. the ability to sort a grid by the values in a field).  

If we can use tags in a grid filter, it seems like it might not be much of a stretch to allow the use of a tag in a field equation or auto-assign rule (i.e. if an item is tagged with a specific tag, then assign to the field value A, if not, then value B).