When can you call yourself doctor? I just passed my dissertation defense. This means there is nothing left but paperwork to get my doctorate. Among the various congratulations I've received a couple have termed me: Dr. My assumption was that I'm not really a doctor until I go through the graduation ceremony, but now I'm wondering. Is there a convention as to when exactly I can call myself Doctor?
Writing as an Administrator: 

It is appropriate to use the title when you are a graduate, ie, when the degree is conferred either in notice by letter or by ceremony (which ever comes first).  Prior to that your status is that of a graduand.  If you've been using the work-title PhD Candidate you might consider changing to PhD Graduand to indicate this status: that you're awaiting conferral but you've met the substantive criteria for fulfillment of your degree.  Additional source: Swinburn on Postnominals.

Congratulations by the way!