Form API file upload field default value implementation using AJAX/AHAH
Unfortunately, depsite the vast amount of answers posted to this question, there was no easy way to do this.  In the end, I had to check when the form was being displayed if the form already had content, and if so, if the form had a file attached.  I then displayed a different value rather than the form field and made a validation workaround due to the fact that the file upload was mandatory.

My advice to others with similar problems: if you're using this as an user profile field, avoid FAPI and stick to CCK and Content Profile.  Although it can be a pain sometimes, all of this functionality is already taken care of.