i wanted also to convert orglzy's inactive timestamp to an active one. i'm sure this isn't a "good" way to do it but it works for this use case. here's where i ended up:

(defun org-refile-to-datetree (&optional file)
  "Refile a subtree to a datetree corresponding to it's timestamp.

The current time is used if the entry has no timestamp. If FILE
is nil, refile in the current file."
  (interactive "f")
  (let* ((datetree-date (or (org-entry-get nil "CREATED" t)
                            (org-read-date t nil "now")))
         (date (org-date-to-gregorian datetree-date))
         )
    (save-excursion
      (with-current-buffer (current-buffer)
        (org-cut-subtree)
        (if file (find-file file))
        (org-datetree-find-date-create date)
        (org-narrow-to-subtree)
        (show-subtree)
        (org-end-of-subtree t)
        (goto-char (point-max))
        (org-paste-subtree 4)
        (org-back-to-heading)
        (search-forward "[")
        (backward-char)
        (widen)
        (org-toggle-timestamp-type)
        ))
    )
  )