safari makes canvas pages complicated, because modules/fb expects two cookies to be set (the Drupal session, and the fbs_APIKEY cookie set by facebook). Sometimes, safari will not accept either one.

Try the very latest copy of the 3.x branch. Just today I checked in some changes to session handling. Things are not perfect on safari, yet.

Also I recommend starting an issue on drupal.org. It's easier to help there.