An interesting addition to the Summer ’12 release is the pilot of separate OWD settings on custom objects for external users. For example, a custom object may have public read-write sharing for internal users and private sharing for external users. In many cases this will simplify the sharing model significantly as the rules below have historically applied to all users including portal users and guest users.

Is there a user who can’t see all records == private
Is there a user who can’t edit all records == public read-only
else == public read-write

Applying the above OWD rules across internal and external users typically results in an overly private model for internal users and a resultant proliferation of performance-expensive sharing rules. The new separation of concerns here is a great improvement for custom objects, but not standard objects (hopefully this is on the roadmap).

Seemingly simple, yet powerful functional enhancements such as this that reduce the amount of custom configuration applied to an org are to be applauded.