The keyboard shortcut Alt+Shift+D inserts a DATE field using the currently selected default date format. If you want just date text rather than a field, you have to immediately unlink it with Ctrl+Shift+F9. Greg's macro is more efficient, however, because
it inserts the date as text to begin with.
A possibly better approach for letters, however, is to use a letter template that includes the CREATEDATE field. Whenever you create a new document based on this template, the field will be updated to the current date and will never update thereafter (the
template retains the date it was created, but new documents have their own creation dates).
For more on creating letter templates, see http://WordFAQs.mvps.org/Letterhead.htm.