I'm running safari 4.0.3 with default security settings, nothing unusual, nothing special for cookies. If I navigate to your app while logged into facebook, it looks exactly the same as if I visit it directly by entering http://apps.facebook.com/drupalforfacebook without being logged into facebook. That is I can see your content but there is no menu to create content or leave comments. If I click on the tour link I make it to http://apps.facebook.com/drupalforfacebook/node/7/dff_tour/identity where I am asked to authorize the app. After clicking the authorize button it gets stuck in an infinite redirect to the same page. If I manually enter other nodes (ie http://apps.facebook.com/drupalforfacebook) those nodes infinitely refresh too. In Firefox 3.0.5, the app still doesn't ask me to authorize until I take the tour, but there is no problem with the redirect. Once I authorize the app it logs me in and shows my facebook name as the DFF username. After adding the the app with firefox, I can go back to safari and everything seems to work.

Procedure for testing:
1. In firefox: Facebook -> Application Settings -> Drupal Facebook -> X (remove app)
2. In firefox: Facebook -> Account -> Logout
3. In safari: Delete cookies
4. In safari: Facebook -> Login
5. In safari: apps.facebook.com/drupalforfacebook
6. In safari: click "tour" link
7. In safari: continue tour to http://apps.facebook.com/drupalforfacebook/node/7/dff_tour/identity
8. After authorizing the app the infinite redirect should occur