I think (but could be quite wrong) that the letters were sent out to waitlistees that were chosen for acceptance some time ago. At least one letter was written in April though it wasn't mailed until mid-late May. The people who are being called now most likely represent those chosen after May 15th, and any acceptances from now on will likely be given by phone. Just my take on things.
BTW, you might wan't to post anything related to the GW waitlist in the "GW waitlistees ... post here" thread: <a href="http://forums.studentdoctor.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=011210;p=1" target="_blank">http://forums.studentdoctor.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=011210;p=1</a>
This makes it easier to keep up on all the relevant information, and probably will garner more responses to any questions you may have.
Hang in there!