It’s quite common to make the title of a document, the same of the filename. A little headache arrives when that filename includes a version number, and the title is supposed to have it as well (for printouts).
Rather then having to update it everytime you save as a new version, I use the filename fields as the title, so it’s “automatic”.
However, this still needs a right click on the field and ‘update’, or, click on the field and hit F9.
Printing the file will also automatically update this field.
But if you really want it updated automatically, apparently MS has the suggestion for a macro:
http://support.microsoft.com/kb/832897