Fletch says:
It's definitely a person, though I'm
not sure I can tell if it's a man or a woman. I think I saw a photograph of
a woman in a similar position, but I can't find it now. It'd be cool to do a
side by side comparison, eh? It's totally NOT cgi either, it's a real person.
Source:
http://www.file83.com/yeo/hi_rez_zoom_person_waving_nWTC.avi
(Higher rez)
also
http://www.file83.com/yeo/zoom_person_waving_nWTC.wmv